### 1 Money-Pump Arguments

It’s 1955. You’ve been offered a full professorship with a salary of $5,000. The dean calls you to his office to go over the final details. On his desk lie three contracts – labelled *A*, *B*, *C*.

“As you know, your current offer (contract *A*) is a full professorship at $5,000,” the dean says, handing you the contract. “Yet … a little birdie told me you prefer an assistant professorship at $6,000 (contract *C*) since it pays a lot more. Potentially, I could offer you the assistant professorship. Potentially.”

The dean rubs his thumbs over his index and middle fingers – the gesture for money. Message received. You slip him $20.

“It’s a pleasure to offer you the assistant professorship,” the dean says, handing you contract *C* in exchange for *A*. “Even so, I’ve been informed you prefer an associate professorship at $5,500 (contract *B*) since it’s more prestigious for just a little bit less money. Might that be worth something to you?”

The dean rubs his fingers. All right. You slip him another $20.

“It’s, ahem, a pleasure to offer the associate professorship,” the dean says, handing you contract *B* in exchange for *C*. “Still, I’ve heard you prefer your first offer (contract *A*) since it’s still more prestigious for only slightly less money.”

The dean rubs his fingers: You slip him another $20.

“It is a real pleasure to *re*offer you the full professorship,” the dean says, handing you contract *A* in exchange for *B*. “Well deserved.”

Once more, you got your first offer, but now you’ve lost $60 to the dean – who, by the way, is only getting started. The dean nods towards contract *C*, rubbing his fingers.Footnote ^{1}

You’ve been reduced to a money pump!Footnote ^{2} You’ve become the dean’s private cash dispenser. A victim of your own mind, you were brought to ruin by the structure of your preferences. You preferred *A* to *B*, *B* to *C*, and *C* to *A*. This cycle of preferences left you open to blatant exploitation. So such cyclic preferences must, it seems, be irrational.

Arguments of this kind let us demonstrate that some alleged requirement of rationality really is a requirement of rationality. A *money-pump argument* for some alleged requirement of rationality consists of an argument that otherwise rational agents who violate the requirement would in some possible situation end up paying for something they could have kept for free even though they knew in advance what decision problem they were facing.

*final outcome*be a description of the world that captures everything that the agent cares about.Footnote

^{3}Let a

*prospect*be a probability distribution over all potential final outcomes.Footnote

^{4}(And let a

*sure prospect*be a prospect with a single possible final outcome.Footnote

^{5}) Expected Utility Theory, then, is the theory that prospects are preferred in accordance with an expected-utility function:Footnote

^{6}

*Expected Utility Theory* Let Ω be the set of possible final outcomes and
${p}_{X}\left(o\right)$ be the probability of outcome *o* in prospect *X*. Then there is a real-valued function *u* such that, for all prospects *X* and *Y*, it holds that *X* is at least as preferred as *Y* if and only if

Rather than this general form, we will be concerned with Expected Utility Theory restricted to prospects with *finite support* – that is, prospects with a finite number of final outcomes with positive probability. Given this restriction, Expected Utility Theory is entailed by the following basic axioms:Footnote ^{7}

Completeness (Section 3)

Transitivity (Section 4.1)

The strong strict-preference version of Independence (Section 5.3)

Continuity (Section 6)

Our main task will be to show, with the help of money-pump arguments, that these axioms are requirements of rationality. For the first three axioms, we will find compelling money-pump arguments.Footnote ^{8} But, for Continuity, we will only be able to find an argument that is almost a money-pump argument.

Money-pump arguments are often dismissed due to a number of influential objections – for example: (i) that you could rationally avoid being money pumped if you use foresight, (ii) that you could rationally avoid being money pumped if you are resolute and stick to a plan, and (iii) that money-pump arguments prove too much, because, in some cases with infinite series of trade offers, even agents who conform to Expected Utility Theory are exploitable.

We will rebut these and other objections. While foresight blocks the standard version of the money-pump argument, there are other versions that work for agents who use foresight (Section 2.1). Once the resolute approach is spelled out in detail, the problems with escaping money pumps by being resolute become apparent (Section 7). And agents who conform to Expected Utility Theory avoid exploitation even in cases with infinite series of trade offers, as long as they use foresight (Section 8).

We won’t start with the axioms of Expected Utility Theory, however. Rather, we’ll start with the most discussed money-pump argument: the argument that rational preferences are acyclic.Footnote ^{9}

### 2 Acyclicity

#### 2.1 Three-Step Acyclicity

Consider having a cup of coffee with one, two, or three lumps of sugar. Suppose that you can’t taste the difference between a cup with one lump and a cup with two lumps. Nor can you taste the difference between a cup with two lumps and a cup with three lumps. And, when you can’t taste any difference, you prefer having less sugar (to keep your intake down). Still, you can taste the difference between a cup with one lump and a cup with three lumps – and, due to your sweet tooth, you prefer the latter.Footnote ^{10}

*A*,

*B*, and

*C*be the sure prospects of having a cup with one, two, and three lumps of sugar respectively. You prefer

*A*to

*B*,

*B*to

*C*, and

*C*to

*A*. Let ‘ $X\succ Y$’ denote that

*X*is (strictly) preferred to

*Y*.Footnote

^{11}Then we can state your preferences as follows:Footnote

^{12}

(1) $A\succ B\succ C\succ A$.

^{13}

*Three-Step Acyclicity* If
$X\succ Y\succ Z$, then it is not the case that
$Z\succ X$.

All violations of Three-Step Acyclicity have the same form as the preferences in (1). So, to show that Three-Step Acyclicity is a requirement of rationality, all we need to show is that preferences of the kind in (1) are irrational.

The standard version of the money-pump argument runs as follows.Footnote^{14}Suppose that you start off with

*A*. An exploiter offers you a trade from

*A*to

*C*. Since you prefer

*C*to

*A*, you accept this offer. Then, after this first trade, you are offered a second trade from

*C*to

*B*. Since you prefer

*B*to

*C*, you also accept this second trade. Finally, after the second trade, you are offered a third trade from

*B*to ${A}^{-}$, where ${A}^{-}$ is just like

*A*except that you have less money and

(2) $A\succ {A}^{-}\succ B$.

We can let the payment be monetary to fit the exploitation framing, but the main point is that
${A}^{-}$ is less preferred than *A* by being certainly inferior with respect to some dimension you care about and the same in other respects.Footnote ^{15} Let a *souring* of a prospect *X* be a prospect that is just like *X* except that it is certainly inferior in a dimension the agent cares about. (And let a *sweetening* of a prospect *X* be a prospect that is just like *X* except that it is certainly superior in a dimension the agent cares about.)

^{16}

*Unidimensional Continuity of Preference* If
$X\succ Y$, then there is a prospect
${X}^{-}$ such that (i)
${X}^{-}$ is a souring of *X* and (ii)
$X\succ {X}^{-}\succ Y$.

The idea is that, if you (strictly) prefer *X* to *Y*, you must prefer *X* with some margin. So there should be some, perhaps minimal, amount you’re willing to pay to get *X* rather than *Y*.Footnote ^{17} This is what blocks trivial responses to the money-pump argument where the agent avoids exploitation by preferring not to pay for anything.Footnote ^{18}

Now, since you prefer
${A}^{-}$ to *B*, you also accept the third trade. So you end up with
${A}^{-}$ (that is, you pay for *A*) when you could have kept *A* for free.

In this example, you followed an approach known as *myopic choice* – that is, you assessed each choice in isolation, as if it were the only choice you would ever make.Footnote ^{19} We distinguish myopic choice from *naive choice*, which is to (i) consider the prospects of all available plans and assess which of these prospects are choice-worthy in a choice between all of them and (ii) choose in accordance with a plan to end up with one of these choice-worthy prospects – without taking into consideration whether you would later depart from that plan.Footnote ^{20}

*A*, ${A}^{-}$,

*B*, and

*C*) and assess which of these prospects are choice-worthy. But how do you choose among three or more prospects if you have cyclic preferences over those prospects? Consider the Maximization Rule:Footnote

^{21}

*The Maximization Rule* It is rationally permitted to choose a prospect *X* if and only if there is no feasible prospect *Y* such that
$Y\succ X$.

*A*, ${A}^{-}$,

*B*, and

*C*, since each prospect is less preferred than another prospect. We avoid this problem with the following alternative rule:Footnote

^{22}

*The Uncovered-Choice Rule* It is rationally permitted to choose a prospect *X* if and only if there is no feasible prospect *Y* such that
$Y\succ X$ and, for all feasible prospects *Z*, it holds that
$Y\succ Z$ if
$X\succ Z$.

*C*. Yet, since you prefer

*C*to

*A*, you may, plausibly, also prefer

*C*to a souring of

*A*.Footnote

^{23}So we could plausibly suppose

(3) $C\succ {A}^{-}$.

Then, given the Uncovered-Choice Rule, the only prospects you would be rationally permitted to choose from *A*,
${A}^{-}$, *B*, and *C* are *A*, *B*, and *C*. Note that
${A}^{-}$ is ruled out because *A* is preferred to
${A}^{-}$ and to every option that
${A}^{-}$ is preferred to.

Even so, does the naive approach avoid exploitation in this case? It does not. Naive choice combined with the Uncovered-Choice Rule still allows you to accept the first two trades, since you regard *B* as choice-worthy. So you are rationally permitted to choose in accordance with the plan to accept the first two trades to get *B*. Then, when you face the third offer of trading from *B* to
${A}^{-}$, the Uncovered-Choice Rule allows you to choose
${A}^{-}$ (because *A* is no longer the prospect of some available plan). So adopting naive choice does not save you from exploitation.

The standard version of the money-pump argument isn’t very convincing, however. In order for a money-pump argument to be compelling, the agent must know in advance what decision problem they face – that is, they must know the whole exploitation set-up in advance.Footnote ^{24} The exploiter must not rely on any knowledge about what will happen that is unavailable to the agent, because being exploited by someone who knows more than you need not be a sign of irrationality.Footnote ^{25} But, if you know the whole exploitation set-up in advance, you can use foresight to see that some of the trades aren’t in your interest.Footnote ^{26} To see how, consider the decision tree of the standard money-pump set-up in Figure 1, which we can call the *Standard Money Pump*.

Here, the three squares represent choice nodes corresponding to the three consecutive trade offers. You accept a trade by going up at the corresponding choice node, and you turn the trade down by going down.Footnote ^{27} The item on the upper left of each square is what you get if you accept the trade, and the item on the lower left is what you keep if you turn the trade down (that is, what you give up if you accept the trade). The preferences stated below the decision tree are the agent’s preferences, which are held constant throughout.Footnote ^{28} So, in this case and in the other cases we’ll consider, agents do not revise their preferences during the decision problem.Footnote ^{29}

*backward induction*is to predict what you would choose at later choice nodes and to take those predictions into account when you choose at earlier nodes.Footnote

^{30}First, consider the trade at node 3. At this node, you have a choice between ${A}^{-}$ and

*B*. And, since you prefer ${A}^{-}$ to

*B*, you would accept the trade to ${A}^{-}$ at node 3. (The choices that are prescribed by backward induction are marked by the thicker lines in the decision tree.) This assumes that the only thing that should guide your choice at a node is your preference between the still feasible options; we accept the following principle:Footnote

^{31}

*Decision-Tree Separability* The rational status of the options at a choice node does not depend on other parts of the decision tree than those that can be reached from that node.

In what follows, we’ll take Decision-Tree Separability for granted (until Section 7, where we take on challenges to this kind of separability).

Using backward induction at node 2, we take into account the prediction that
${A}^{-}$ would be chosen at node 3. Given this prediction, accepting the trade at node 2 effectively results in your final holding being
${A}^{-}$ whereas turning it down results in your final holding being *C*. Since you prefer *C* to
${A}^{-}$, you would turn down the trade at node 2.

And, taking this prediction into account at node 1, we find that accepting the trade at node 1 effectively results in your final holding being *C* whereas turning it down results in your final holding being *A*. Since you prefer *C* to *A*, you accept the trade at node 1. Hence, using backward induction, you will end up with *C* after accepting the first trade and then turning down the second. So you avoid paying for something you could have kept for free. And so the standard money-pump argument is blocked.Footnote ^{32}

Nevertheless, we can revise the exploitation set-up so that it works against people who use backward induction.

One way to do so is to repeat the trade offers in case they are rejected but with no more than three trade offers in total, as in the decision problem in Figure 2, the *Money Pump with Repeated Offers*.Footnote ^{33}

At any of the final nodes (that is, nodes 3, 4, 6, and 7), you would accept the trades you are offered. In other words, you would go up at each of these nodes.

Using backward induction at nodes 2 and 5, you take into account the prediction that you would accept each of the final trades. So the choice at node 2 is effectively between
${A}^{-}$ (accepting the trade) and *B* (turning it down). Since you prefer
${A}^{-}$ to *B*, you would accept the trade at node 2 (that is, you would go up to node 3). Likewise, the choice at node 5 is effectively between *B* (accepting the trade) and *C* (turning it down). Since you prefer *B* to *C*, you would accept the trade at node 5 (that is, you would go to node 6).

Using backward induction at node 1, you take all of these predictions into account. So the choice at node 1 is effectively between
${A}^{-}$ (accepting the trade) and *B* (turning it down). Since you prefer
${A}^{-}$ to *B*, you accept this first trade. So you go up to node 2 and then up to node 3, where you finally choose
${A}^{-}$. And then you end up with
${A}^{-}$ even though you could have kept *A* for free.

Still, this argument relies upon a contentious form of backward induction. Specifically, one may challenge our assumption that you would choose rationally and retain your trust in your future rationality even at choice nodes that could only be reached by irrational choices.Footnote ^{34} To see the problem, suppose that you predict not only rational choices but also some irrational choices in the Money Pump with Repeated Offers. We mark these predicted choices at nodes that would follow irrational choices by thick dashed lines in Figure 3.

Taking these predictions into account at node 1, the choice at that node is effectively between *C* (accepting the trade) and *B* (turning it down). Since you prefer *B* to *C*, it’s rationally required that you turn down the trade at node 1. And the move to node 2 is irrational. So the predicted irrational choices would only be made at nodes that follow irrational choices. Hence, in order to rule out these predictions, we must assume that you would choose rationally and retain your trust in your future rationality even at nodes that follow irrational choices. But it’s implausible that you would be rationally required to retain your trust in your future rationality at nodes where you’ve already made irrational choices.Footnote ^{35}

This worry about backward induction does not apply to decision problems that are BI-terminating. A decision problem is *BI-terminating* if and only if the choices that are prescribed by backward induction are terminal, that is, the prescribed choices are not followed by any potential further choice nodes.Footnote ^{36} To defend the prescriptions of backward induction in BI-terminating decision problems, we only need to assume that, at nodes that can be reached without making any irrational choices, you retain (i) your rationality and (ii) your trust in your rationality at nodes that can be reached without making any irrational choices. While the money pumps we’ve considered so far are not BI-terminating, the *Upfront Money Pump* in Figure 4 is BI-terminating.Footnote ^{37}

Since you prefer *C* to *A*, you would accept the trade at node 3. Using backward induction, you would take this prediction into account at node 2. Given the predicted choice at node 3, the choice at node 2 is effectively between *B* (accepting the trade) and *C* (turning it down). Since you prefer *B* to *C*, you would accept the trade at node 2. And, taking this prediction into account at node 1, the initial choice is effectively between
${A}^{-}$ (accepting the trade) and *B* (turning it down). Since you prefer
${A}^{-}$ to *B*, you accept the trade at node 1 and end up with
${A}^{-}$ even though you could have kept *A* for free.

This backward-induction argument assumes that you would also make rational choices and retain your trust in your future rationality at nodes that follow irrational choices. But, since the Upfront Money Pump is BI-terminating, the choices that are prescribed by backward induction can be defended by a more compelling argument without this assumption. We only need the weaker assumption that, at nodes that can be reached without making any irrational choices, you retain (i) your rationality and (ii) your trust in your rationality at nodes that can be reached without making any irrational choices. The argument takes the form of a proof by contradiction.Footnote ^{38}

Assume that all three choice nodes can be reached without making any irrational choices. So, at these nodes, you retain your rationality and your trust in your rationality at these nodes. Accordingly, you would accept the trade at node 3, since you prefer *C* to *A*. Taking this into account at node 2, the choice at node 2 is effectively between *B* and *C*. And, since you prefer *B* to *C*, it’s irrational to turn down the trade at node 2. This contradicts our assumption that all three choice nodes can be reached without making any irrational choices.

Next, assume that nodes 1 and 2 can be reached without making any irrational choices. So, at these nodes, you retain your rationality and your trust in your rationality at these nodes. Since we have already shown that it’s irrational to go down at (at least) one of nodes 1 and 2, it must be irrational to go down at node 2. So it must be rationally required to accept the trade at node 2.Footnote ^{39} Accordingly, you would accept the trade at node 2. Taking this into account at node 1, the choice at node 1 is effectively between
${A}^{-}$ and *B*. And, since you prefer
${A}^{-}$ to *B*, it’s irrational to turn down the trade at node 1. This contradicts our assumption that nodes 1 and 2 can be reached without making any irrational choices.

Hence it’s irrational to turn down the trade at node 1. So it’s rationally required to accept the first trade from *A* to
${A}^{-}$. And so you end up with
${A}^{-}$ when you could have kept *A* for free.

For this argument, we only assumed that, at nodes that can be reached without making any irrational choices, you retain (i) your rationality and (ii) your trust in your rationality at nodes that can be reached without making any irrational choices.

If your preferences are robust under a uniform monetary sweetening (for instance, a penny), we can extend the Upfront Money Pump so that you pay an arbitrarily high amount. Consider the decision problem in Figure 5, the *Ruinous Upfront Money Pump*.Footnote ^{40}

With the same backward-induction argument as before, we find that it’s rationally required to accept the first trade. So you end up paying over $1,000,000 for *A* when you could have kept *A* for free. And, as the Ruinous Upfront Money Pump is still a BI-terminating decision problem, we can defend the prescriptions of backward induction in this case with the same minimal assumptions we relied on for the Upfront Money Pump.

It may be objected that choosing
${A}^{-}$ at node 1 in the Upfront Money Pump is not a sign of irrationality, since the sequence of choices leading to *A* isn’t available in the relevant sense at that node. The idea being that the sequence of choices leading to *A* isn’t securable at node 1, because, at that node, you can’t make your future self make those choices.Footnote ^{41}

But the target of the money-pump argument isn’t your choice at the first node, which does seem rational *given your preferences*. The target is the structure of your preferences. And the reason why you can’t secure the sequence of choices that leads to *A* (even though you can secure the choice of
${A}^{-}$) is the cyclic structure of your preferences.Footnote ^{42}

It may next be objected that, even though you prefer
${A}^{-}$ to *B*, you could still prefer *B* to a more specific version of
${A}^{-}$, such as
${A}^{-}$-*when-you-could-have-kept*-*A*.Footnote ^{43} And, if so, you could be rationally required to turn down the trade at node 1 in the Upfront Money Pump even though you prefer
${A}^{-}$ to *B*. Hence, by individuating final outcomes (and thereby prospects) finely enough, you may avoid exploitation even though you have the preferences in (1).

This objection assumes that there are no restrictions on how we may individuate final outcomes. But, if there were no such restrictions, requirements of rationality such as Three-Step Acyclicity would be compatible with the rationality of any sequence of choices, because any alleged violation would disappear given some more fine-grained individuation of final outcomes.Footnote ^{44} To get around this problem, we need to adopt a principle of individuation for final outcomes.

^{45}

*The Principle of Individuation by Rational Indifference* Final outcomes *x* and *y* should be treated as the same if and only if it is rationally required to be indifferent between the sure prospects of *x* and *y*.

Of course, if it were rationally permitted not to be indifferent between *A* and
${A}^{-}$-*when-you-could-have-kept*-*A*, these prospects may still be treated as distinct. But, as we shall see in Section 7, it is irrational not to be indifferent between such prospects.Footnote ^{46}

Does the Principle of Individuation by Rational Indifference violate the transitivity of identity (the principle that, if
$X=Y=Z$, then *X* = *Z*)? Consider again the example of having a cup of coffee with one, two, or three lumps of sugar: You can’t tell the difference between a cup with one lump and a cup with two lumps. Nor can you tell the difference between a cup with two lumps and a cup with three lumps. But you can tell the difference between a cup with one lump and a cup with three lumps. As before, let *A*, *B*, and *C* be the sure prospects of having a cup with one, two, and three lumps of sugar respectively. When you can’t tell the difference between two prospects, it’s arguably rationally required to be indifferent between them. So then it’s rationally required to be indifferent between *A* and *B* and between *B* and *C*. Yet it seems rational to have a preference between *A* and *C*. So we find that
$A=B=C\ne A$, which violates the transitivity of identity. This objection, however, is blocked if we allow indirect ways of telling the difference between the options. You can tell that a cup with one lump (*A*) tastes noticeably different from a cup with three lumps (*C*), whereas a cup with two lumps (*B*) does not taste noticeably different from a cup with three lumps (*C*). This difference in how they compare to *C* is a noticeable difference between *A* and *B*. And, in the same manner (changing what needs to be changed), you can distinguish *B* and *C*.Footnote ^{47}

It may also be objected that you could resist exploitation in the Upfront Money Pump if you adopt self-regulation. Basically, *self-regulation* forbids, if it can be avoided, choosing options that may be followed by a rationally permitted sequence of choices that has a prospect that you would not have chosen in a direct choice between the prospects of all available plans.Footnote ^{48} Following the Uncovered-Choice Rule, you wouldn’t choose
${A}^{-}$ in a direct choice between *A*,
${A}^{-}$, *B*, and *C*. So self-regulation prescribes that you turn down the first trade in the Upfront Money Pump. And then you avoid exploitation.

Nevertheless, cyclic preferrers who adopt self-regulation are still vulnerable to the arguments we used to defend the choices that backward induction prescribes in the Upfront Money Pump. This is the main problem with the self-regulation defence of cyclicity.Footnote ^{49} Moreover, this objection to self-regulation works in all the decision problems we will rely on in our overall money-pump argument for Expected Utility Theory.

(4) $A\succ {A}^{-}\succ B\succ {B}^{-}\succ C\succ {C}^{-}\succ A$,

where
${A}^{-}$,
${B}^{-}$, and
${C}^{-}$ are sourings of *A*, *B*, and *C* respectively.

Consider the decision problem in Figure 6, the *Three-Way Money Pump*.Footnote ^{50}

In the Three-Way Money Pump, you would go up at each of nodes 2, 3, and 4. So, no matter what you choose at node 1, you end up paying for something that you could have had for free. For instance, if you go up at node 1 and up at node 2, then you end up with
${A}^{-}$ when you could have had *A* (by going down at node 1 and down at node 4).

This exploitation scheme doesn’t work, however. The exploiter cannot use it without potentially giving up something. The problem is that you can’t be relied on to choose a certain option at node 1, so you might end up paying for any one of *A*, *B*, and *C* no matter what your initial holding were. For instance, if you start with *A*, the exploiter potentially needs to trade you one of *B* and *C* to get your money. And then the scheme looks less profitable for the exploiter.Footnote ^{51} It also looks less irrational for you, because you may end up with a final holding that you prefer to your initial holding.Footnote ^{52} If you start off with *A* and end up with
${B}^{-}$ or with
${C}^{-}$, you do not pay for what you could have *kept* for free.

It may be objected that whether some behaviour is a sign of irrationality shouldn’t depend on how profitable it is for an exploiter. Isn’t the mere fact that you chose
${X}^{-}$ when you could have had *X* a sufficient sign of irrationality? If so, our task constructing money pumps would be much easier. But, if that were a sign of irrationality, then the money-pump argument would prove too much, since clearly rational preferences would be irrational in some cases with an infinite series of trades (as we shall see in Section 8). Being exploited by giving exploiters a free lunch seems worrying in a separate way from merely making a sequence of choices that has a prospect that is less preferred than the prospect of some alternative sequence of choices.

(5) $A\succ {A}^{-}\succ {A}^{--}\succ B\succ {B}^{-}\succ {B}^{--}\succ C\succ {C}^{-}\succ {C}^{--}\succ A$,

where ${A}^{--}$, ${B}^{--}$, and ${C}^{--}$ are sourings of ${A}^{-}$, ${B}^{-}$, and ${C}^{-}$ respectively.

Let a *state of nature* be a description of the world that resolves all of the agent’s uncertainty except that it leaves open what the agent will choose.Footnote ^{53} Let an *event* be a set of states of nature.Footnote ^{54} Let
$\neg E$ be the complement of event *E*, that is, the event that *E* does not occur. Let
$E\&{E}^{*}$ be the intersection of events *E* and
${E}^{*}$, that is, the event that both *E* and
${E}^{*}$ occur. Let a *partition* of states of nature be a set of events such that (i) each event in the set includes at least one state of nature and (ii) each state of nature is a member of exactly one event in the set. And let a *gamble* be a distribution of prospects over a partition of states of nature.

*E*

_{1}and

*E*

_{2}are two independent chance events such that

*E*

_{1}occurs with a $1/3$ probability and

*E*

_{2}occurs with a $1/2$ probability. And consider the gambles

*G*

_{1}, ${G}_{1}^{-}$, and

*G*

_{2}whose outcomes depend on

*E*

_{1}and

*E*

_{2}:

E_{1} | $\neg {E}_{1}\&{E}_{2}$ | $\neg {E}_{1}\&\neg {E}_{2}$ | |
---|---|---|---|

( $1/3$) | ( $1/3$) | ( $1/3$) | |

G_{1} | A | B | C |

${G}_{1}^{-}$ | ${A}^{-}$ | ${B}^{-}$ | ${C}^{-}$ |

G_{2} | ${C}^{--}$ | ${A}^{--}$ | ${B}^{--}$ |

Here, $\{{E}_{1},\neg {E}_{1}\&{E}_{2},\neg {E}_{1}\&\neg {E}_{2}\}$ is a partition of states of nature.

We adopt the following requirement of rationality:Footnote^{55}

*The Weak Principle of Equiprobable Unidimensional Dominance* If there are sets of events
$\{{E}_{1},{E}_{2},\dots \}$ and
$\{{E}_{1}^{*},{E}_{2}^{*},\dots \}$ such that these sets are partitions of states of nature and, for all
$i=1,2,\dots $, it holds that (a) *E*_{i} has the same probability as
${E}_{i}^{*}$, (b) the outcome of gamble
${G}^{*}$ given
${E}_{i}^{*}$ is a souring of the outcome of gamble *G* given *E*_{i}, and (c) the outcome of *G* given *E*_{i} is preferred to the outcome of
${G}^{*}$ given
${E}_{i}^{*}$, then
$G\succ {G}^{*}$.

(6) ${G}_{1}\succ {G}_{1}^{-}\succ {G}_{2}$, and ${G}_{1}\succ {G}_{2}$.

Now, consider the decision problem in Figure 7, the *Self-Regulation Money Pump*.

Here, the circles represent chance nodes where the way forward depends on a chance event. Chance nodes 2 and 4 go up if and only if *E*_{1} occurs. And chance nodes 3 and 6 go up if and only if *E*_{2} occurs.

You start off with gamble *G*_{1}. At choice node 1, you are offered a trade from *G*_{1} to
${G}_{1}^{-}$. If you turn down the trade at node 1, you would get an offer to trade from the outcome of *G*_{1} to the outcome of *G*_{2} after the chance events have resolved. This second trade offer would be offered to you at one of choice nodes 5, 7, and 8.

At each of nodes 5, 7, and 8, you would accept the second trade offer. Using backward induction or self-regulation, you take these predictions into account at node 1. And then the choice at node 1 is effectively between
${G}_{1}^{-}$ (accepting the trade) and *G*_{2} (turning it down).

So, taking future choices into account at node 1, neither of the prospects that are effectively available at that node – that is,
${G}_{1}^{-}$ and *G*_{2} – would be chosen in a direct choice between the prospects of all available plans given the Uncovered-Choice Rule, because *G*_{1} is preferred to both of them. Hence self-regulation forbids neither
${G}_{1}^{-}$ nor *G*_{2}, since you can’t avoid choosing an option with a prospect that you would not have chosen in a direct choice between the prospects of all available plans. So it seems that, even if you rely on self-regulation, you should accept the first trade (since you prefer
${G}_{1}^{-}$ to *G*_{2}). But then you end up with
${G}_{1}^{-}$ when you could have kept *G*_{1} for free. Hence this money pump isn’t blocked by self-regulation, and it makes you pay for what you could have kept for free.Footnote ^{56}

#### 2.2 Acyclicity

So far, we have only considered arguments that rational preferences conform to Three-Step Acyclicity. We haven’t considered the following, more general, requirement:Footnote^{57}

*Acyclicity* If
${X}_{1}\succ {X}_{2}\succ \dots \succ {X}_{n}$, then it is not the case that
${X}_{n}\succ {X}_{1}$.

Yet we can show that Acyclicity is a requirement of rationality in much the same way as Three-Step Acyclicity.

Suppose that you violate Acyclicity by having the following, arbitrarily large, cycle of preferences:(7) ${A}_{1}\succ {A}_{2}\succ \dots \succ {A}_{n}\succ {A}_{1}$.

(8) ${A}_{1}\succ {A}_{1}^{-}\succ {A}_{2}\succ \dots \succ {A}_{n}\succ {A}_{1}$,

where
${A}_{1}^{-}$ is a souring of *A*_{1}. We can then extend the Upfront Money Pump to handle this arbitrarily large cycle. Consider the decision problem in Figure 8, the *Upfront Acyclicity Money Pump*.Footnote ^{58}

Here, you are first offered the opportunity to trade from *A*_{1} to
${A}_{1}^{-}$. If you were to turn down that offer, you would be offered a trade from *A*_{1} to *A*_{2}. Then, if you were to turn down that offer too, you would be offered a trade from *A*_{1} to *A*_{3}, and so on until you would be offered a final trade from *A*_{1} to *A*_{n}.

By backward induction, we find (in the same manner as in the Upfront Money Pump) that it’s rationally required to accept the first trade. So you end up with
${A}_{1}^{-}$ when you could have kept *A*_{1} for free.

To spell out the assumptions of the money-pump arguments, we will rely on the notion of plans being available. Let a *plan* at a node *n* be a specification of what to choose at each choice node that can be reached from *n* while following the specification. Let us say that one *follows a plan* from a node *n* if and only if, for each choice node
${n}^{*}$ that can be reached from *n* while choosing in accordance with the plan, one would choose in accordance with that plan if one were to face
${n}^{*}$. Moreover, let us say that one *intentionally follows a plan* from a node *n* if and only if one follows the plan from *n* and, for all nodes
${n}^{*}$ such that
${n}^{*}$ can be reached from *n* by following the plan, if one were to face
${n}^{*}$, one would either form or have formed at
${n}^{*}$ an intention to choose in accordance with the plan at every choice node that can both be reached from *n* and be reached from
${n}^{*}$ by following the plan. Finally, let us say that *a plan is available* at a node *n* if and only if the plan can be intentionally followed from *n*.Footnote ^{59}

^{60}

*The Principle of Unexploitability* If (i)
${X}^{-}$ is a souring of *X*, (ii)
$X\succ {X}^{-}$, (iii), at node *n*, it holds that *P* and
${P}^{-}$ are two available plans such that *P* is the only available plan that amounts to walking away from all offers by an exploiter and the prospect of following *P* is *X* and the prospect of following
${P}^{-}$ is
${X}^{-}$, and (iv) one knows what decision problem one faces at *n*, then one does not follow
${P}^{-}$ from *n*.

This is the main assumption of money-pump arguments: that it is irrational to knowingly pay (in some currency you care about) for what you could have kept for free.

We also assume the following principle:*The Principle of Preferential Invulnerability* If there is a possible situation where having a certain combination of preferences forces one to violate a requirement of rationality, then there is a requirement of rationality that rules out that combination of preferences in all possible situations.

Given this principle, rational preferences must not lead to any conflicts with any requirements of rationality in any possible situation. The underlying idea is that there’s no *rational luck*.Footnote ^{61} Whether you are rational shouldn’t depend on what situation you happen to find yourself in. So whether it’s unlikely that you will ever face a money-pump set-up is irrelevant.Footnote ^{62}

Backward induction at nodes that can be reached without making irrational choices

The Principle of Unexploitability

Unidimensional Continuity of Preference

Decision-Tree Separability

The possibility of the Upfront Acyclicity Money Pump

The Principle of Preferential Invulnerability

We need the possibility of the Upfront Acyclicity Money Pump, since the Principle of Preferential Invulnerability only covers possible situations. This assumption is substantial, since it may be rejected if some outcomes cannot occur in the relevant sequential patterns. We will also take it as part of the description of the Upfront Money Pump to handle this arbitrarily large cycle. Consider the decision problem in the Upfront Money Pump (and the other decision problems we will discuss) that, at each node, all plans at that node are available. We do not, however, assume that the money-pump situations we discuss are likely to arise. As conceived here, the money-pump argument against cyclic preferences does not aim to show that having acyclic preferences is useful, or that cyclic preferences are likely to have bad effects.Footnote ^{63}

One worry about the Upfront Acyclicity Money Pump (and the other money pumps we’ll discuss) is that that decision problem is impossible if the violating preferences are non-practical preferences. Consider the following example.Footnote ^{64} Let *A* be the sure prospect of staying at home. Let *B* be the sure prospect of going to Rome. And let *C* be the sure prospect of going mountaineering. Here, your preferences may plausibly be sensitive to what alternatives are available. Suppose that you prefer *A*-*when-the-only-alternative-is*-*B* to *B*, prefer *B* to *C*, and prefer *C* to *A*-*when-the-only-alternative-is*-*C*. These transitive (and plausible) preferences are *practical* in the sense that, for each pairwise preference, there is a possible choice between the compared prospects. Accordingly, preferences are *non-practical* if and only if it is not the case that, for each pairwise preference, there is a possible choice between the compared prospects. But suppose that you also prefer *C* to *A*-*when-the-only-alternative-is*-*B*. This makes your preferences cyclical. So, to defend Acyclicity, we need to show that these preferences are irrational. The additional preference, however, is non-practical, since there is no possible choice between *C* and *A*-*when-the-only-alternative-is*-*B*. So, for these cyclic preferences, the Upfront Acyclicity Money Pump is impossible.

To handle this problem, we rely on the Principle of Individuation by Rational Indifference.Footnote ^{65} The fine-grained prospects *A*-*when-the-only-alternative-is*-*B* and *A*-*when-the-only-alternative-is*-*C* differ not only in what the alternative would be but also in that you are a coward in the latter but not in the former. Plausibly, the difference you are rationally permitted to care about is not the difference in alternative but the difference in cowardice. So you are rationally required to be indifferent between *A*-*when-the-only-alternative-is*-*B* and *A*-*and-not-being-a-coward*. The Principle of Individuation by Rational Indifference then entails that these prospects should be treated as the same. Crucially, it’s possible to have a choice between *A*-*and-not-being-a-coward* and *C*. So there is a possible instance of the Upfront Acyclicity Money Pump for your following practical preference cycle: *A*-*and-not-being-a-coward* is preferred to *B*, *B* is preferred to *C*, and *C* is preferred to *A*-*and-not-being-a-coward*.Footnote ^{66}

^{67}

*One-Step Acyclicity* It is not the case that
$X\succ X$.

^{68}

*Two-Step Acyclicity* If
$X\succ Y$, then it is not the case that
$Y\succ X$.

*X*is at least as preferred as

*Y*.Footnote

^{69}We then adopt the following definition of that

*X*is

*preferred*to

*Y*:Footnote

^{70}

$X\succ Y{=}_{\text{df}}X\succsim Y$ and it is not the case that $Y\succsim X$.

Given this definition of strict preference, violations of One- and Two-Step Acyclicity are impossible.Footnote ^{71}

### 3 Completeness

Consider having an apple or having an orange. Suppose that, given their different qualities, you can’t compare these options: you do not prefer one of them to the other, yet you’re not indifferent between them.Footnote ^{72}

^{73}

*Completeness*
$X\succsim Y$ or
$Y\succsim X$.

*X*is

*indifferent*to

*Y*, defined as follows:Footnote

^{74}

$X\sim Y{=}_{\text{df}}X\succsim Y$ and $Y\succsim X$.

*preferential gap*between

*X*and

*Y*, defined as follows:Footnote

^{75}

$X\parallel Y{=}_{\text{df}}$ it is neither the case that $X\succsim Y$ nor the case that $Y\succsim X$.

*A*be the sure prospect of having the apple, and let

*B*be the sure prospect of having the orange. Then we can state your preferences as follows:

(9) $A\parallel B$.

As defined, indifference and preferential gaps are both symmetrical relations.Footnote ^{76} So, in the absence of any further requirements of rationality, it’s hard to make any practical distinction between these relations. If we, for example, take some preferences that satisfy Expected Utility Theory and replace all indifference relations with preferential gaps, the resulting preferences would be no more exploitable than the original preferences. These new preferences, which violate Completeness, would be practically equivalent to the original preferences, which satisfy Completeness. So we need some practically relevant difference between indifference and preferential gaps or the distinction won’t matter – robbing Completeness of practical substance.

^{77}

*Weak Insensitivity to Souring*If $X\parallel Y$, then

there is a prospect ${X}^{-}$ such that (i) ${X}^{-}$ is a souring of

*X*and (ii) $X\succ {X}^{-}\parallel Y$ orthere is a prospect ${Y}^{-}$ such that (i) ${Y}^{-}$ is a souring of

*Y*and (ii) $Y\succ {Y}^{-}\parallel X$.

The idea is that this robustness to sourings holds for preferential gaps but not for indifference. But couldn’t two prospects be related by a preferential gap even though any souring of either prospect breaks the gap? For the purpose of our discussion, we can treat such prospects as being indifferent, because the main assumption we will make about indifference for the money-pump argument for Transitivity (specifically, the souring approach in Section 4.1) is that any souring would break the indifference between prospects. Hence we treat Weak Insensitivity to Souring as a stipulation rather than an assumption.

We will, however, make the substantial assumption that the following principle is a requirement of rationality:*Symmetry of Souring Sensitivity* If (i)
${X}^{-}$ is a souring of *X* and (ii)
$X\succ {X}^{-}\parallel Y\parallel X$, then there is a prospect
${Y}^{-}$ such that (i)
${Y}^{-}$ is a souring of *Y* and (ii)
$Y\succ {Y}^{-}\parallel X$.

If you have a preferential gap between *X* and *Y*, there must be some kind of perplexity about the comparison of these prospects. This perplexity should plausibly be symmetrical – in the sense that, if the perplexity swallows sourings on one side, it should also do so on the other.Footnote ^{78}

*Strong Insensitivity to Souring* If
$X\parallel Y$, then there is a prospect
${X}^{-}$ such that (i)
${X}^{-}$ is a souring of *X* and (ii)
$X\succ {X}^{-}\parallel Y$.

(10) $A\succ {A}^{-}\parallel B\parallel A$,

where
${A}^{-}$ is a souring of *A*.

Now, consider the (potential) money pump for preferential gaps in Figure 9, the *Single-Souring Money Pump*.Footnote ^{79}

At node 2, you have a preferential gap between the two options,
${A}^{-}$ and *B*. It seems, therefore, that it’s neither irrational to choose
${A}^{-}$ nor irrational to choose *B*.Footnote ^{80}

So, at node 1, backward induction does not let you rule out any of the options being chosen (or picked) at node 2.Footnote ^{81} But, if you can’t rule out that any one of the options at node 2 would be chosen, it’s unclear how you should take that choice into account at node 1.Footnote ^{82} One of the options at node 2, *B*, is no less preferred than the prospect of going down at node 1, *A*. So it may seem that it shouldn’t be irrational to go up at node 1. And, if it isn’t irrational to go up at node 1, it seems that it isn’t irrational to both go up at node 1 and go up at node 2. But, if you go up at both node 1 and node 2, you end up with
${A}^{-}$ when you could have kept *A* for free, which violates the Principle of Unexploitability.

The Single-Souring Money Pump, on this interpretation, is an example of a non-forcing money pump. A money-pump set-up is *forcing* if and only if the agent is rationally required, at each step of the set-up, to going along with the exploitation. A money-pump set-up is *permitting* if and only if, at each step of the set-up, the agent is rationally permitted to go along with the exploitation. A money-pump set-up is *non-prohibiting* if and only if, at each step of the set-up, the agent is not rationally prohibited from going along with the exploitation. Finally, a money-pump set-up is *non-forcing* if and only if it is non-prohibiting and, at some step of the set-up, the agent is not rationally required to go along with the exploitation.Footnote ^{83}

While non-forcing money pumps may be problematic for the agent, they are implausible as exploitation schemes.Footnote ^{84} Since it’s not irrational to choose *B* at node 2 of the Single-Souring Money Pump, you might turn down the second trade. And, if you do, you end up with *B* and the exploiter has given up *B* for *A*. So, even though the Single-Souring Money Pump does offer the exploiter an opportunity to potentially get your money for free, the exploiter might end up merely trading you *B* for *A* – which need not be in their interest (nor is it contrary to your interest).

^{85}

*The Principle of Rational Decomposition* If an agent, whose credences and preferences are not rationally prohibited, makes a sequence of choices which violates a requirement of rationality, then some of those choices are rationally prohibited.

This principle is plausible. If no choice in a sequence of choices is irrational, it’s hard to see where the irrationality of the sequence would be coming from (given that your credences and preferences aren’t irrational). If you didn’t violate any requirement of rationality at any point during an interval, it seems that you didn’t violate any requirement of rationality during the interval.

Suppose that, contrary to the Principle of Rational Decomposition, you make an irrational sequence of choices where no choice is irrational and your credences and preferences are not irrational. Then this sequence is only ruled out by diachronic requirements of rationality – that is, the sequence is irrational but no requirement of rationality rules out your credences, preferences, or choices at any moment during the sequence. So, during the interval in which you made this sequence of choices and violated these diachronic requirements, there was no moment at which you did something that violated any requirement of rationality. This robs the prohibition of the sequence of any practical relevance, because we cannot make atemporal choices. So, without any help from other requirements, how could these diachronic requirements guide you away, practically, from completing the sequence?Footnote ^{86} It is tempting to say that, at the final choice node where you have a choice whether to complete the irrational sequence of choices, these diachronic requirements would be violated if you were to make that final choice of the sequence; so that final choice must be rationally prohibited.Footnote ^{87} But, if so, we have no violation of the Principle of Rational Decomposition.Footnote ^{88}

Does the Principle of Rational Decomposition conflict with the Principle of Unexploitability? You might violate the latter by following a dominated plan, where each choice seems rational given your preferences. But that violation of the Principle of Unexploitability need not violate the Principle of Rational Decomposition, since your preferences could be irrational. And, if your preferences are irrational, you violate a requirement of rationality at each moment you have those preferences.

Given the Principle of Rational Decomposition and the Principle of Unexploitability, it’s either irrational to go up at node 1 or irrational to go up at node 2 (assuming that your credences and preferences aren’t irrational). Could we plausibly claim that it’s irrational to go up at node 2?

A potential way to do so is to adopt forward induction. With *forward induction*, one deliberates under the assumption that one’s past choices were rational.Footnote ^{89} If the choice to go up at node 1 were rational (or at least not irrational), it seems that you must choose *B* at node 2. You must choose *B* at node 2, because, if you instead choose
${A}^{-}$, then the choice to go up at node 1 was effectively a choice of
${A}^{-}$ when you could have kept *A*. And to choose
${A}^{-}$ when you could keep *A* is irrational, contradicting the assumption that the choice at node 1 wasn’t irrational.Footnote ^{90}

But forward induction based on your own choices is implausible. The trouble lies in explaining why the rational status of your choice at node 1 should matter to you at node 2.Footnote ^{91} The reason why going up at node 1 would be irrational if you were to also go up at node 2 is that you then end up with
${A}^{-}$ when you could still have kept *A* at node 1. At node 2, however, *A* is no longer feasible. That *A* wasn’t chosen at node 1 is now (at node 2) just a sunk cost.Footnote ^{92} The only thing that should guide your choice at node 2 is your preference between the still feasible options. That is, we rely on Decision-Tree Separability.

Since it’s implausible that choosing ${A}^{-}$ would be irrational at node 2, let us turn to the other alternative. Can we plausibly claim that it’s irrational to go up at node 1? We can.

Suppose that you went up at both node 1 and node 2. Then you end up with
${A}^{-}$, which is less preferred than something you could have chosen at node 1, namely, *A*. What choice do you regret? It should be the first choice. With the second choice you merely turned down a prospect that you don’t prefer to the prospect you ended up with. So you have no reason to regret the second choice. Looking back, it’s with the first choice that you turned down what you prefer to your final holding.Footnote ^{93}

*credences*about what would be chosen at later choice nodes. I’m only denying that what it’s rational to choose now could depend on what will actually happen in the future. We accept the following principle:

*The Principle of Future-Choice Independence* The rational status of an option at a choice node and the rational status of the agent’s credences and preferences at that node do not depend on what would in fact be chosen at later choice nodes.

Note that this principle does not conflict with backward induction, since backward induction only relies on *predictions* about what would be chosen at later choice nodes and not on what would in fact be chosen.

Since going up at node 1 is irrational if you also go up at node 2, it follows, by the Principle of Future-Choice Independence, that it’s irrational to go up at node 1 no matter what you would choose at node 2.

This argument that it’s irrational to go up at node 1 can be generalized. We will do so now to show that the following principle is a rational requirement:*Minimal Unidimensional Precaution* If (i)
${X}^{-}$ is a souring of *X*, (ii)
$X\succ {X}^{-}$, (iii) it is not the case that
$Y\succ {X}^{-}$, (iv) node *n* is a choice between node
${n}^{*}$ and *X*, (v) node
${n}^{*}$ is a choice between
${X}^{-}$ and *Y*, and (vi) one knows at node *n* what decision problem one faces, then one chooses *X* at node *n*.

^{94}

*The Irrationality of Single Sourings* If (i)
${X}^{-}$ is a souring of *X*, (ii)
$X\succ {X}^{-}$, (iii) node *n* is a choice between node
${n}^{*}$ and *X*, (iv) node
${n}^{*}$ is a choice between
${X}^{-}$ and *Y*, and (v) one knows at node *n* what decision problem one faces, then the sequence of choices consisting in choosing node
${n}^{*}$ at node *n* and
${X}^{-}$ at node
${n}^{*}$ violates a requirement of rationality.

(11) $A\succ {A}^{-}$, and it is not the case that $B\succ {A}^{-}$.

Assume, for proof by contradiction, that you did not violate a requirement of rationality even though you violated Minimal Unidimensional Precaution. So your credences and preferences are not rationally prohibited. Now, regardless of whether you will in fact choose
${A}^{-}$ at node 2, we may consider what the rational status of your choices would be if you were to choose
${A}^{-}$ at node 2. Note first that, even if you actually choose *B* at node 2, your credences and preferences at nodes 1 and 2 would be the same as they actually are at these nodes if you were to choose
${A}^{-}$ at node 2.Footnote ^{95} From the Principle of Future-Choice Independence, it then follows that your credences and preferences wouldn’t be rationally prohibited if you were to choose
${A}^{-}$ at node 2. So, if you were to go up at both choice nodes, it follows, by the Irrationality of Single Sourings, that this sequence of choices would be irrational. So, by the Principle of Rational Decomposition, at least one of your choices would be irrational (since your credences and preferences are not rationally prohibited). But, given Decision-Tree Separability, your choice at node 2 cannot be irrational. Hence, if you were to go up at both choice nodes, it would be your choice to go up at node 1 that would be irrational. Then, by the Principle of Future-Choice Independence, it follows that the rational status of your choice at node 1 cannot depend on what you choose at node 2. Hence the choice to go up at node 1 must be irrational regardless of whether you would choose *B* at node 2. So the choice to go up at node 1 is irrational. And, since this argument can be given for all violations of Minimal Unidimensional Precaution (changing what needs to be changed), it follows that Minimal Unidimensional Precaution is a requirement of rationality.

Hence – from Decision-Tree Separability, the Irrationality of Single Sourings, the Principle of Future-Choice Independence, and the Principle of Rational Decomposition – we have derived that Minimal Unidimensional Precaution is a requirement of rationality.

This result may seem puzzling in case you’re certain what you would choose at a future node even though the choice at that node is a choice between two options that are related by a preferential gap. You could, for instance, be certain that you will follow a particular tie-breaker rule for resolving choices where neither option is preferred to the other.Footnote ^{96} It may be objected that, if you are certain at node 1 that you would follow a tie-breaker rule at node 2 which favours choosing *B*, then you seem rationally permitted to go up at node 1, even though this violates Minimal Unidimensional Precaution. If you are certain in this manner what you would choose in the future, you do not (according to this objection) need precaution.

But note that the argument for Minimal Unidimensional Precaution makes no assumptions about what your credences are. So the argument’s assumptions should be no less plausible in case you’re certain at node 1 that *B* would be chosen at node 2. The source of puzzlement, here, may be the existence of an objection in the vicinity which does block the argument for Minimal Unidimensional Precaution: It may seem that, if you are certain at node 1 that you would follow a particular tie-breaker rule at node 2, then it would be irrational not to follow that tie-breaker rule at node 2. This suggestion contradicts one of the argument’s assumptions – namely, Decision-Tree Separability. So this suggestion would block argument. We will take Decision-Tree Separability for granted, however, until Section 7. So we postpone our discussion of this objection until then.Footnote ^{97}

*rationally allowed outcome*of an option

*X*be a prospect of an available plan consisting in choosing

*X*followed by choices that are not irrational. At node 1, there’s no potential upside to going up, because no rationally allowed outcome of going up is preferred to the prospect of going down. But there is a potential downside to going up, because one of the rationally allowed outcomes of going up is less preferred than the prospect of going down. So it’s irrational to go up at node 1. This argument supports a precautionary version of backward induction:

According to *precautionary backward induction*, it is irrational to choose an option *X* over an option *Y* if there is a rationally allowed outcome of *X* (that is, a prospect of an available plan consisting in choosing *X* followed by choices that are not irrational) that is less preferred than some rationally allowed outcome of *Y* and there is no rationally allowed outcome of *Y* that is less preferred than some rationally allowed outcome of *X*.

Precautionary backward induction entails (given Decision-Tree Separability) that going up is irrational at node 1. Likewise, if Minimal Unidimensional Precaution is (as we have argued) a requirement of rationality, we also find that going up is irrational at node 1. So the attempted exploitation in the Single-Souring Money Pump is blocked.

Hence following either Minimal Unidimensional Precaution or precautionary backward induction makes you invulnerable to the Single-Souring Money Pump, but, as we shall see, it does not save you from two variations of that decision problem.

From (10), we have, by Strong Insensitivity to Souring,(12) $A\succ {A}^{-}\parallel B\succ {B}^{-}\parallel A\parallel B$,

where
${B}^{-}$ is a souring of *B*.

Consider the decision problem in Figure 10, the *Dual-Souring Money Pump*.Footnote ^{98}

No matter what you choose at node 1 in this case, you will end up at a node where (given Decision-Tree Separability) it is not irrational to pay for something you could have had for free. And neither Minimal Unidimensional Precaution nor precautionary backward induction will help you.

But the Dual-Souring Money Pump is not a plausible exploitation scheme, because the exploiter might end up merely trading you *B* for *A* or
${A}^{-}$. This wouldn’t necessarily be in the exploiter’s interest, nor would it be contrary to your interest. (Hence the Dual-Souring Money Pump suffers from the same problem as the Three-Way Money Pump.)

(13) $A\succ {A}^{-}\succ {A}^{--}\parallel B\succ {B}^{-}\parallel A\parallel B\parallel {A}^{-}$,

where ${A}^{--}$ is a souring of ${A}^{-}$.

Now, consider the decision problem in Figure 11, the *Precaution Money Pump*.

At node 4, neither option is less preferred than the other, so it’s neither irrational to choose *A* nor irrational to choose
${B}^{-}$.

Taking this into account at node 3, we find that turning down the trade at node 3 has a rationally allowed outcome (
${B}^{-}$) that is less preferred than the prospect of accepting the trade (*B*) but there is no rationally allowed outcome of turning the trade down that is preferred to the prospect of accepting the trade. So, by precautionary backward induction, you would accept the trade at node 3. Alternatively, we could rely on Minimal Unidimensional Precaution, which also prescribes going up at node 3 (since
${B}^{-}$ is a souring of *B* and you do not prefer *A* to
${B}^{-}$).

Taking this prediction into account at node 2, the choice at that node is effectively between
${A}^{--}$ and *B*. Since neither of these options is less preferred than the other, it’s neither irrational to accept nor irrational to turn down the trade at node 2.

Taking this into account at node 1, we find that turning the trade down has a rationally allowed outcome (
${A}^{--}$) that is less preferred than the prospect of accepting the trade (
${A}^{-}$) but turning the trade down cannot lead (via choices that aren’t irrational) to a prospect that is preferred to the prospect of accepting the trade. So, by precautionary backward induction, you accept the trade at node 1. Alternatively, we could rely on Minimal Unidimensional Precaution, which also prescribes going up at node 1 (since
${A}^{--}$ is a souring of
${A}^{-}$ and you do not prefer *B* to
${A}^{--}$). Hence you accept the first trade and end up with
${A}^{-}$, even though you could have kept *A* for free.

The Precaution Money Pump isn’t BI-terminating, since going down at node 2 is allowed by backward induction and would be followed by the choice at node 3. Even so, we can still defend the prescriptions of precautionary backward induction or Minimal Unidimensional Precaution combined with standard backward induction without assuming that you retain your rationality and your trust in your rationality at nodes that can only be reached by irrational choices. As before, we only need the assumption that, at nodes that can be reached without making any irrational choices, you retain (i) your rationality and (ii) your trust in your rationality at nodes that can be reached without making any irrational choices.

Assume, for proof by contradiction, that each of nodes 2–4 can be reached without making any irrational choices. Then, at nodes 1–4, you retain your rationality and your trust in your rationality at these nodes. Hence, at node 4, you might choose either of *A* and
${B}^{-}$, since neither prospect is preferred to the other (so neither option is rationally prohibited at node 4). Taking this into account with precautionary backward induction, we find that it’s irrational to turn down the trade at node 3. Alternatively, we could rely on Minimal Unidimensional Precaution, which also entails that it’s irrational to turn down the trade at node 3 (since
${B}^{-}$ is a souring of *B* and you do not prefer *A* to
${B}^{-}$). But this conclusion, that it’s irrational to turn down the trade at node 3, contradicts our assumption that each of nodes 2–4 can be reached without making any irrational choices.

Assume next, for proof by contradiction, that each of nodes 2 and 3 can be reached without making any irrational choices. Then, at nodes 1–3, you retain your rationality and your trust in your rationality at these nodes. Since we have already shown that it’s irrational to go down at (at least) one of nodes 1–3, it must be irrational to go down at node 3. So it’s rationally required to accept the trade at node 3. So you would accept the trade at node 3. Then, taking this prediction into account at node 2, the choice at that node is effectively between
${A}^{--}$ (accepting the trade) and *B* (turning it down). So you might choose either to accept or to turn down the trade at node 2, since neither of
${A}^{--}$ and *B* is preferred to the other (so neither choice at node 2 is irrational). Taking this prediction into account at node 1 with precautionary backward induction, we find that it’s irrational to turn down the trade at node 1. This is so, because one of the rationally allowed outcomes of turning the trade down (
${A}^{--}$) is less preferred than the prospect of accepting the trade (
${A}^{-}$) but none of the rationally allowed outcomes of turning the trade down (that is, neither
${A}^{--}$ nor *B*) is preferred to the prospect of accepting the trade (
${A}^{-}$). Alternatively, we could rely on Minimal Unidimensional Precaution, which also entails that it’s irrational to turn down the trade at node 1 (since
${A}^{--}$ is a souring of
${A}^{-}$ and you do not prefer *B* to
${A}^{--}$). But this conclusion, that it’s irrational to go down at node 1, contradicts our assumption that each of nodes 2 and 3 can be reached without making any irrational choices.

Finally, assume, for proof by contradiction, that node 2 can be reached without making any irrational choices. Then, at nodes 1 and 2, you retain your rationality and your trust in your rationality at these nodes. Since we have already shown that it’s irrational to go down at (at least) one of nodes 1 and 2, it must be irrational to go down at node 2. So you would accept the trade at node 2. Taking this prediction into account at node 1, we find that the choice at that node is effectively between
${A}^{-}$ (accepting the trade) or
${A}^{--}$ (turning it down). Since you prefer
${A}^{-}$ to
${A}^{--}$, it is irrational to turn the trade down and go to node 2. But this contradicts our assumption that node 2 can be reached without making any irrational choices. Hence it is irrational to go down at node 1. So it’s rationally required to accept the trade at node 1. So you accept the trade from *A* to
${A}^{-}$ at node 1. And then you end up with
${A}^{-}$ even though you could have kept *A* for free. And we managed to show this without assuming that you would make rational choices at nodes that can only be reached by irrational choices.

Backward induction at nodes that can be reached without making irrational choices

The Principle of Unexploitability

Symmetry of Souring Sensitivity

Decision-Tree Separability

The Irrationality of Single Sourings

The possibility of the Precaution Money Pump

The Principle of Future-Choice Independence

The Principle of Preferential Invulnerability

The Principle of Rational Decomposition

Weak Insensitivity to Souring

Moreover, since we only relied on the Irrationality of Single Sourings, the Principle of Future-Choice Independence, and the Principle of Rational Decomposition in the derivation of Minimal Unidimensional Precaution, we could drop these assumptions if we assume Minimal Unidimensional Precaution as a requirement of rationality. Likewise, since Minimal Unidimensional Precaution as a requirement of rationality is equivalent (given standard backward induction) to precautionary backward induction in the Precaution Money Pump, we may also drop Minimal Unidimensional Precaution in this argument if we assume precautionary backward induction at nodes that can be reached without making irrational choices.

If we also assume that the preferences in (13) are robust for small differences in money, we can create a ruinous version of the Precaution Money Pump by iteratively extending the scheme backwards so that you have an initial choice between (i) paying the exploiter a large sum of money to go away and (ii) not paying the exploiter and then face a long sequence of iterations of the Precaution Money Pump with gradually smaller payments.

If Completeness is a requirement of rationality, does this mean that you are required to form an opinion about all possible pairs of options?Footnote ^{99} This depends on what, more precisely, we take preference relations to be. A preference relation between *X* and *Y* is, I suggest, a disposition to have a certain psychologically real mental ranking of *X* and *Y if you were to compare X and Y*.Footnote ^{100} So you do not have to have a psychologically real mental ranking of all possible options, which would be impossible for our limited minds. It’s sufficient to have a psychologically real algorithm for arriving at a mental ranking of the options in case you were to compare them. A calculator may be a helpful analogy. A calculator does not have all sums stored for all pairs of numbers it may be asked to add up. What it has is a (storage-wise) relatively small algorithm for how to calculate these sums when needed. Completeness, given the suggested understanding of preference relations, doesn’t demand anything less feasible than that.Footnote ^{101}

### 4 Transitivity

#### 4.1 Transitivity

Consider, once more, having a cup of coffee with one, two, or three lumps of sugar. Suppose, as before, that you can taste the difference between a cup with one lump and a cup with three lumps but you can neither taste the difference between a cup with one lump and a cup with two lumps nor taste the difference between a cup with two lumps and a cup with three lumps. This time, however, you only care about taste. Accordingly, you’re indifferent between having a cup with one lump and having a cup with two lumps and indifferent between having a cup with two lumps and having a cup with three lumps. And, due to your sweet tooth, you prefer having a cup with three lumps to having a cup with one lump.Footnote ^{102}

^{103}

*Transitivity* If
$X\succsim Y\succsim Z$, then
$X\succsim Z$.

(1) $A\succ B\succ C\succ A$.

(14) $A\succsim B\succsim C\parallel A$.

^{104}

(15) $A\succ B\succ C\sim A$.

(16) $A\succ B\sim C\sim A$.

The money pumps we discussed in Section 2 don’t work for agents who have the preferences in (15) or (16). In the Upfront Money Pump, for example, agents with these preferences do not prefer *C* to *A*, so they aren’t rationally required to accept the trade from *A* to *C*. And then the argument falls apart.

*A*and

*C*may be chosen at node 3, agents with the preferences in (16) can rely on the following dominance version of backward induction – which, given Completeness, is equivalent to precautionary backward induction:

According to *dominance backward induction*, it is irrational to choose an option *X* over an option *Y* if there is a rationally allowed outcome of *X* (that is, a prospect of an available plan consisting in choosing *X* followed by choices that are not irrational) that is less preferred than some rationally allowed outcome of *Y* and every rationally allowed outcome of *Y* is at least as preferred as every rationally allowed outcome of *X*.

Dominance backward induction faces much the same worry as Minimal Unidimensional Precaution and precautionary backward induction, namely, that it seems less plausible in case you’re sure that, if you were to choose *X*, you wouldn’t make a sequence of choices that has a less preferred prospect than any rationally allowed outcome of *Y*. But much the same response applies here too (see Section 3).

Given dominance backward induction, the agent with the preferences in (16) is rationally required to turn down the trade at node 2, because the rationally allowed outcomes of doing so (that is, *A* and *C*) are both at least as good as the rationally allowed outcome of accepting the trade (*B*) and one of the rationally allowed outcomes of turning the trade down (*A*) is preferred to the rationally allowed outcome of accepting it. Plausibly, *C* is not only at least as preferred as *A* but also at least as preferred as
${A}^{-}$. Therefore, taking into account at node 1 what may be chosen at later nodes, we find (given dominance backward induction) that it’s rationally required to turn down the trade at node 1. Hence the earlier money-pump argument is blocked.

One way to extend the money-pump argument for Acyclicity so that it also works for agents who have the preferences in (15) or (16) is the *souring approach*, which is to convert those acyclic, non-transitive preferences into cyclic preferences of the kind in (1) by breaking the indifferences with the help of sourings.Footnote ^{105}

(17) $A\succ {A}^{-}\succ B$,

where
${A}^{-}$ is a souring of *A*.

^{106}

*Unidimensional IP-Transitivity* If (i)
${Y}^{-}$ is a souring of *Y* and (ii)
$X\sim Y\succ {Y}^{-}$, then
$X\succ {Y}^{-}$.

This requirement is plausible. Since a souring of *Y* does not improve *Y* in any dimension the agent cares about, a souring of *Y* should tip the scale between the two indifferent prospects *X* and *Y* in favour of the unsoured *X*. Nevertheless, Unidimensional IP-Transitivity may seem to assume, to some extent, the point at issue in an argument that Transitivity is a requirement of rationality. We will deal with this worry shortly.

(18) $C\succ {A}^{-}$.

(19) $B\succ C\succ {A}^{-}\succ B$.

We have derived cyclic preferences of the kind in (1), which can be shown to be irrational with the Upfront Money Pump.

Next, suppose that you have the preferences in (16). The first two steps proceed as before. From (16), we have, by Unidimensional Continuity of Preference,(20) $A\succ {A}^{-}\succ B$,

*A*. From (16) and (20), we have, by Unidimensional IP-Transitivity,

(21) $C\succ {A}^{-}$.

(22) $C\succ {C}^{-}\succ {A}^{-}$,

*C*. From (16) and (22), we have, by Unidimensional IP-Transitivity,

(23) $B\succ {C}^{-}$.

(24) $B\succ {C}^{-}\succ {A}^{-}\succ B$.

As before, we have derived cyclic preferences of the kind in (1), which can be shown to be irrational with the Upfront Money Pump.

Hence we have a money-pump argument that Transitivity is a requirement of rationality, and this argument relies on the following requirements of rationality:Backward induction at nodes that can be reached without making irrational choices

Completeness (defended in Section 3)

The Principle of Unexploitability

Unidimensional Continuity of Preference

Unidimensional IP-Transitivity

Decision-Tree Separability

The possibility of the Upfront Money Pump

The Principle of Preferential Invulnerability

Still, as mentioned, Unidimensional IP-Transitivity is a special case of Transitivity. So it assumes, at least in part, the point at issue in an argument for Transitivity. Many of the alleged counter-examples to Transitivity would also be counter-examples to Unidimensional IP-Transitivity. For instance, let *A* be a free trip to Austin, let *B* be a free trip to Boston, and let
${B}^{-}$ be the trip to Boston at a cost of $1.Footnote ^{107} It may seem rationally permitted to be indifferent between *A* and *B* and between *A* and
${B}^{-}$ while one prefers *B* to
${B}^{-}$. If we rebut alleged counter-examples of this kind with the help of Unidimensional IP-Transitivity, we assume the point at issue.Footnote ^{108}

But there is a better response to these alleged counter-examples. If your indifference between two prospects is insensitive to sourings in this manner, then your indifference would have the same problematic insensitivity to sourings as preferential gaps. And then you would be open to a variation of the Precaution Money Pump (from Section 3), replacing preferential gaps with indifference.

Even so, there are other ways to amend the money-pump argument for Transitivity. The following*eventwise approach*makes use of dominance in terms of events. That is, we assume the following requirement of rationality:Footnote

^{109}

*The Strong Principle of Eventwise Dominance* If there is a set of events such that (i) the set is a partition of states of nature, (ii), given each event *E* in the set, the outcome of gamble *G* given *E* is at least as preferred as the outcome of gamble
${G}^{*}$ given *E*, and (iii), in some positive-probability event
${E}^{*}$ in the set, the outcome of *G* given
${E}^{*}$ is preferred to the outcome of
${G}^{*}$ given
${E}^{*}$, then
$G\succ {G}^{*}$.

This principle avoids the earlier worry about IP-Transitivity. Note that the standard, alleged, counter-examples to Transitivity would not be counter-examples to the Strong Principle of Eventwise Dominance. So the Strong Principle of Eventwise Dominance does not assume the point at issue against these alleged counter-examples. So it does not assume the point at issue in an argument for Transitivity.

(Yet, since we will rely on Transitivity for the argument for the strong strict-preference version of Independence in Section 5.3, it may seem that the Strong Principle of Eventwise Dominance assumes, in part, the point at issue in an argument for Independence. In this respect, the souring approach is better. Still, in Section 5.1, we will rebut the commonly claimed counter-examples to the Strong Principle of Eventwise Dominance without relying on Transitivity.)

Suppose that you violate Transitivity by having the preferences in one of (1), (15), and (16). Then you have the following preferences:(25) $A\succ B\succsim C\succsim A$.

*G*

_{1},

*G*

_{2}, and

*G*

_{3}, which have different outcomes in positive-probability events

*E*

_{1},

*E*

_{2}, and

*E*

_{3}that are such that $\{{E}_{1},{E}_{2},{E}_{3}\}$ is a partition of states of nature:

E_{1} | E_{2} | E_{3} | |
---|---|---|---|

G_{1} | A | B | C |

G_{2} | B | C | A |

G_{3} | C | A | B |

^{110}

(26) ${G}_{1}\succ {G}_{2}\succ {G}_{3}\succ {G}_{1}$.

Once more, we have derived cyclic preferences of the kind in (1), which can be shown to be irrational with the Upfront Money Pump.Footnote ^{111}

Backward induction at nodes that can be reached without making irrational choices

Completeness (defended in Section 3)

The Principle of Unexploitability

The Strong Principle of Eventwise Dominance

Unidimensional Continuity of Preference

Decision-Tree Separability

The possibility of the Upfront Money Pump

The Principle of Preferential Invulnerability

Hence we have two alternative arguments for Transitivity, which rely on notably different assumptions.

#### 4.2 Transitivity of Strict Preference

While we have already argued that Transitivity is a requirement of rationality, it is worth investigating whether we can make do with more compelling assumptions if we merely seek to defend the following (logically weaker) requirement of rationality:Footnote^{112}

*Transitivity of Strict Preference* If
$X\succ Y\succ Z$, then
$X\succ Z$.

(1) $A\succ B\succ C\succ A$.

(15) $A\succ B\succ C\sim A$.

(27) $A\succ B\succ C\parallel A$.

(16) $A\succ B\sim C\sim A$.

The preferences in (16) violate Transitivity (that is, transitivity of *at least as preferred as*) but not Transitivity of Strict Preference. This, as we shall see, allows us to make do without not only Unidimensional IP-Transitivity but also the Strong Principle of Eventwise Dominance.

The preferences in (1) violate Acyclicity, and the preferences in (27) violate Completeness. So, given Acyclicity and Completeness, we only need to show that preferences of the kind in (15) are irrational.

Suppose that you have the preferences in (15). From (15), we have, by Unidimensional Continuity of Preference,(17) $A\succ {A}^{-}\succ B$,

where
${A}^{-}$ is a souring of *A*.

*Unidimensional Continuity of Dispreference* If
$X\succ Y$, then there is a prospect
${Y}^{+}$ such that (i)
${Y}^{+}$ is a sweetening of *Y* and (ii)
$X\succ {Y}^{+}\succ Y$.

The underlying idea behind this principle is the same as for Unidimensional Continuity of Preference: if you (strictly) prefer *X* to *Y*, then you must prefer *X* with some margin. So there should be some, perhaps minimal, amount you are willing to forgo to get *X* rather than *Y*.

(28) $B\succ {C}^{+}\succ C$,

where
${C}^{+}$ is a sweetening of *C*.

^{113}

*Unidimensional PI-Acyclicity* If (i)
${X}^{+}$ is a sweetening of *X* and (ii)
${X}^{+}\succ X\sim Y$, then it is not the case that
$Y\succ {X}^{+}$.

Back in Section 4.1, when we assumed Unidimensional IP-Transitivity for the souring approach, we assumed that the souring of one of two indifferent prospects made it less preferred than the other. And we had to rule out that you may still be indifferent between the prospects. Here, we needn’t do so. All we assume is that a sweetening of one of two indifferent prospects does not make it *less* preferred than the other.

(29) It is not the case that $A\succ {C}^{+}$.

(30) ${C}^{+}\succsim A$.

Now, consider the decision problem in Figure 12, the *Strict-Preference Money Pump*.

At node 4, you are both rationally permitted to go up and rationally permitted to go down, since you are indifferent between *A* and *C*.

Taking this into account at node 3, we find that the prospect of going up (
${C}^{+}$) is preferred to one of the rationally allowed outcomes of going down (*C*) and the prospect of going up is at least as preferred as every rationally allowed outcome of going down. So, by dominance backward induction, it is rationally required that you accept the trade at node 3. Alternatively, we could rely on Minimal Unidimensional Precaution, which also prescribes going up at node 3 – since *C* is a souring of
${C}^{+}$ and you do not prefer *A* to *C*.

Taking this into account at node 2, it is rationally required to accept the trade at that node, since you prefer *B* to
${C}^{+}$.

Finally, taking this prediction into account at node 1, it is rationally required to accept the initial trade, since you prefer
${A}^{-}$ to *B*. So you end up with
${A}^{-}$ even though you could have kept *A* for free.

Note that the Strict-Transitivity Money Pump is BI-terminating. So we only need to assume that, at nodes that can be reached without making irrational choices, you retain (i) your rationality and (ii) your trust in your rationality at nodes that can be reached without making irrational choices.

Hence we have a money-pump argument that Transitivity of Strict Preference is a requirement of rationality, and this argument relies on the following requirements of rationality:Acyclicity (defended in Section 2.2)

Backward induction at nodes that can be reached without making irrational choices

Completeness (defended in Section 3)

The Principle of Unexploitability

Unidimensional Continuity of Dispreference

Unidimensional Continuity of Preference

Unidimensional PI-Acyclicity

Decision-Tree Separability

The Irrationality of Single Sourings

The possibility of the Strict-Preference Money Pump

The Principle of Future-Choice Independence

The Principle of Preferential Invulnerability

The Principle of Rational Decomposition

We only need the Irrationality of Single Sourings, the Principle of Future-Choice Independence, and the Principle of Rational Decomposition to derive Minimal Unidimensional Precaution (see Section 3). So we could drop these assumptions if we assume Minimal Unidimensional Precaution as a requirement of rationality. As a requirement of rationality, Minimal Unidimensional Precaution is equivalent (given standard backward induction) to dominance backward induction in the Strict-Preference Money Pump. So we may also drop Minimal Unidimensional Precaution in this argument if we assume dominance backward induction at nodes that can be reached without making irrational choices.

Note that this approach, which makes do with Unidimensional PI-Acyclicity without Unidimensional IP-Transitivity and the Strong Principle of Eventwise Dominance, does not work for the preferences in (16), where *C* is indifferent not only to *A* but also to *B*. If we tried this approach on those preferences, we would see that, once we have sweetened *C*, the sweetening of *C* may be preferred not only to *A* but also to *B*. And then the approach is blocked.

#### 4.3 Strong Acyclicity

We can extend both the souring approach and the eventwise approach to cover violations of the following weakening of Transitivity (and strengthening of Acyclicity):Footnote^{114}

*Strong Acyclicity* If
${X}_{1}\succsim {X}_{2}\succsim \dots \succsim {X}_{n}$, then it is not the case that
${X}_{n}\succ {X}_{1}$.

(31) ${A}_{1}\succ {A}_{2}\succsim \dots \succsim {A}_{n}\succsim {A}_{1}$.

So suppose that you violate Strong Acyclicity by having the preferences in (31).

For the souring approach, consider first the case where your preferences are, more specifically, of the following cyclical kind:(7) ${A}_{1}\succ {A}_{2}\succ \dots \succ {A}_{n}\succ {A}_{1}$.

In that case, we can show that your preferences are irrational with the Upfront Acyclicity Money Pump.

Consider next the remaining case where your preferences over*A*

_{1},

*A*

_{2}, …, and

*A*

_{n}do not contain a cycle of strict preference. That is, your preferences are merely

*weakly cyclical*– that is, you have a cycle of strict preference except that some (but not all) strict-preference relations in the cycle have been replaced by indifference. Find three prospects –

*A*

_{i},

*A*

_{j}, and

*A*

_{k}– that are adjacent in this weak cycle such that

(32) ${A}_{i}\sim {A}_{j}\succ {A}_{k}$.

(33) ${A}_{j}\succ {A}_{j}^{-}\succ {A}_{k}$,

*A*

_{j}. Then, from (32) and (33), we have, by Unidimensional IP-Transitivity,

(34) ${A}_{i}\succ {A}_{j}^{-}\succ {A}_{k}$.

Next, we replace the (32) part of the weak cycle with (34). We repeat this procedure if necessary until we end up with a cycle of strict preference. Finally, we use the Upfront Acyclicity Money Pump to show that these cyclic preferences are irrational.

Hence we have a money-pump argument that Strong Acyclicity is a requirement of rationality, and this argument relies on the following requirements of rationality:Backward induction at nodes that can be reached without making irrational choices

The Principle of Unexploitability

Unidimensional Continuity of Preference

Unidimensional IP-Transitivity

Decision-Tree Separability

The possibility of the Upfront Acyclicity Money Pump

The Principle of Preferential Invulnerability

*G*

_{1},

*G*

_{2}, …, and

*G*

_{n}, which have different outcomes in positive-probability events

*E*

_{1},

*E*

_{2}, …, and

*E*

_{n}that are such that $\{{E}_{1},{E}_{2},\dots ,{E}_{n}\}$ is a partition of states of nature:

E_{1} | E_{2} | … | E_{n} | |
---|---|---|---|---|

G_{1} | A_{1} | A_{2} | … | A_{n} |

G_{2} | A_{2} | A_{3} | … | A_{1} |

$\vdots $ | $\vdots $ | $\vdots $ | $\vdots $ | |

G_{n} | A_{n} | A_{1} | … | ${A}_{n-1}$ |

(35) ${G}_{1}\succ {G}_{2}\succ \dots \succ {G}_{n}\succ {G}_{1}$.

We have derived cyclic preferences of the kind in (7), which can be shown to be irrational with the Upfront Acyclicity Money Pump.

So we have a money-pump argument that Strong Acyclicity is a requirement of rationality, and this argument relies on the following requirements of rationality:Backward induction at nodes that can be reached without making irrational choices

The Principle of Unexploitability

The Strong Principle of Eventwise Dominance

Unidimensional Continuity of Preference

Decision-Tree Separability

The possibility of the Upfront Acyclicity Money Pump

The Principle of Preferential Invulnerability

Notably, unlike the money-pump argument for Transitivity, these money-pump arguments for Strong Acyclicity do not rely on Completeness.Footnote ^{115}

### 5 Independence

Suppose that you prefer $1M ($1,000,000) for sure to a one-in-two chance of $3M, because you don’t want to risk getting nothing. But you also prefer a one-in-three chance of $3M to a two-in-three chance of $1M, because now there is a risk of getting nothing with either lottery and you prefer the prize of the former lottery to the prize of the latter one.Footnote ^{116}

*XpY*be a prospect consisting in a lottery between

*X*and

*Y*such that

*X*occurs with probability

*p*and

*Y*occurs with probability $1-p$, where

*X*and

*Y*are also prospects that are either lotteries themselves or sure prospects.Footnote

^{117}The most straightforward version of Independence can then be stated as follows:Footnote

^{118}

*Independence (the biconditional weak-preference version)* For all probabilities *p* such that
$0<p<1$, it holds that
$X\succsim Y$ if and only if
$XpZ\succsim YpZ$.

^{119}

*Independence (the strong strict-preference version)* For all probabilities *p* such that
$0<p<1$, it holds that, if
$X\succ Y$, then
$XpZ\succ YpZ$.

A challenge to the idea that Independence is a requirement of rationality is that the most straightforward version – the biconditional weak-preference version – conflicts with some seemingly rational preferences, namely, Allais and Ellsberg Preferences. Those preferences, however, can be shown to be irrational with the help of a money-pump argument with fairly weak assumptions (Section 5.1).

Furthermore, there is a money-pump argument, with even weaker assumptions, that the following version of Independence is a requirement of rationality (Section 5.2):Footnote^{120}

*Independence (the weak strict-preference version)* For all probabilities *p* such that
$0<p<1$, it holds that, if
$X\succ Y$, then it is not the case that
$YpZ\succ XpZ$.

But this version of Independence is too weak to characterize Expected Utility Theory together with Completeness, Continuity, and Transitivity. Still, given somewhat stronger assumptions, the money-pump argument for the weak strict-preference version can be extended so that it also works for the strong strict-preference version (Section 5.3). And, with only slightly stronger assumptions, we can show that the biconditional weak-preference version of Independence is a requirement of rationality (Section 5.4).

#### 5.1 Allais, Ellsberg, and Independence for Constant Outcomes

The two most prominent objections to Independence are the Allais Paradox (put forward by Maurice Allais) and the Ellsberg Paradox (put forward by Daniel Ellsberg). These paradoxes directly challenge not only the biconditional weak-preference version of Independence but also the following, logically weaker, requirement:Footnote^{121}

*Independence for Constant Outcomes (the weak strict-preference version)* For all probabilities *p* such that
$0<p<1$, it holds that, if
$XpU\succ YpU$, then it is not the case that
$YpV\succ XpV$.

Violations of this variant of Independence can only be of the following kind:

(36) $ApC\succ BpC$, and $BpD\succ ApD$,

where *p* is a probability such that
$0<p<1$. As we shall see, the Allais Paradox and the Ellsberg Paradox both feature seemingly rational preferences of this kind.

*The Allais Paradox* involves four lotteries. In lottery *L*_{1}, one gets $1M for certain; in lottery *L*_{2}, there is a
$10\%$ probability of getting $5M, an
$89\%$ probability of getting $1M, and a
$1\%$ probability of getting $0; in lottery *L*_{3}, there is an
$11\%$ probability of getting $1M and an
$89\%$ probability of getting $0; and, in lottery *L*_{4}, there is a
$10\%$ probability of getting $5M and a
$90\%$ probability of getting $0:Footnote ^{122}

$1\%$ | $10\%$ | $89\%$ | |
---|---|---|---|

${L}_{1}$ | $1M | $1M | $1M |

${L}_{2}$ | $0 | $5M | $1M |

${L}_{3}$ | $1M | $1M | $0 |

${L}_{4}$ | $0 | $5M | $0 |

*Allais Preferences*:

(37) ${L}_{1}\succ {L}_{2}$, and ${L}_{4}\succ {L}_{3}$.

*A*be a sure prospect of $1M; let

*B*be the prospect of a $10/11$ probability of $5M, otherwise $0; let

*C*be a sure prospect of $1M (just like

*A*); and let

*D*be a sure prospect of $0:

$1/11$ | $10/11$ | |
---|---|---|

A | $1M | $1M |

B | $0 | $5M |

C | $1M | $1M |

D | $0 | $0 |

*p*be $11/100$, then

*L*

_{1}is equivalent to

*ApC*,

*L*

_{2}is equivalent to

*BpC*,

*L*

_{3}is equivalent to

*ApD*, and

*L*

_{4}is equivalent to

*BpD*. So (37) can be stated as follows:

(36) $ApC\succ BpC$, and $BpD\succ ApD$.

Accordingly, Allais Preferences violate the weak strict-preference version of Independence for Constant Outcomes.

*The Ellsberg Paradox*features an urn that is known to contain 30 red balls and 60 balls that are either black or yellow (and this is all that is known with respect to the proportions in the urn). The proportion of black to yellow balls is unknown. A ball will be drawn at random from the urn. Just like the Allais Paradox, the Ellsberg Paradox involves four lotteries. Lottery

*L*

_{1}pays $100 if the ball is red, otherwise $0; lottery

*L*

_{2}pays $100 if the ball is black, otherwise $0; lottery

*L*

_{3}pays $100 if the ball is red or yellow, otherwise $0; and lottery

*L*

_{4}pays $100 if the ball is black or yellow, otherwise $0:Footnote

^{123}

30 balls | 60 balls | ||
---|---|---|---|

Red | Black | Yellow | |

L_{1} | $100 | $0 | $0 |

L_{2} | $0 | $100 | $0 |

L_{3} | $100 | $0 | $100 |

L_{4} | $0 | $100 | $100 |

*Ellsberg Preferences*:

(38) ${L}_{1}\succ {L}_{2}$, and ${L}_{4}\succ {L}_{3}$.

*p*be the unknown probability of

*the ball’s being either red or black*(which, given the agent’s knowledge, is equivalent to

*the ball’s not being yellow*); let

*A*be the prospect of a $1/\left(3p\right)$ probability of $100, otherwise $0; let

*B*be the prospect of a $1-1/\left(3p\right)$ probability of $100, otherwise $0; let

*C*be the sure prospect of $0; and let

*D*be the sure prospect of $100:

$1/\left(3p\right)$ | $1-1/\left(3p\right)$ | |
---|---|---|

A | $100 | $0 |

B | $0 | $100 |

C | $0 | $0 |

D | $100 | $100 |

*L*

_{1}is equivalent to

*ApC*,

*L*

_{2}is equivalent to

*BpC*,

*L*

_{3}is equivalent to

*ApD*, and

*L*

_{4}is equivalent to

*BpD*. So (38) can be stated as follows:

(36) $ApC\succ BpC$, and $BpD\succ ApD$.

Accordingly – just like Allais Preferences – Ellsberg Preferences violate the weak strict-preference version of Independence for Constant Outcomes.

Hence both Allais and Ellsberg Preferences entail preferences of the kind in (36), so they both violate the weak strict-preference version of Independence for Constant Outcomes. Consequently, Allais and Ellsberg Preferences violate the (logically stronger) biconditional weak-preference version of Independence.Footnote ^{124}Footnote ^{125}

As we have seen, both Allais and Ellsberg Preferences violate the weak strict-preference version of Independence for Constant Outcomes. Accordingly, Allais and Ellsberg Preferences are both irrational if this principle is a requirement of rationality. Can we show that it is a requirement of rationality? We can.

Suppose that you violate the weak strict-preference version of Independence for Constant Outcomes by having the preferences in (36). From (36), we have, by Unidimensional Continuity of Preference,(39) $ApC\succ {A}^{-}p{C}^{-}\succ A{}^{--}p{C}^{--}\succ BpC$, and

$BpD\succ {B}^{-}p{D}^{-}\succ B{}^{--}p{D}^{--}\succ ApD$,

where
${A}^{-}p{C}^{-}$ and
${B}^{-}p{D}^{-}$ are sourings of *ApC* and *BpD* respectively and where
$A{}^{--}p{C}^{--}$ and
$B{}^{--}p{D}^{--}$ are sourings of
${A}^{-}p{C}^{-}$ and
${B}^{-}p{D}^{-}$ respectively.

*The Souring Principle* If
${X}^{-}$ is a souring of *X*, then
$X\succ {X}^{-}$.

(40) $A\succ {A}^{-}\succ {A}^{--}$, $B\succ {B}^{-}\succ {B}^{--}$, and $C\succ {C}^{-}\succ {C}^{--}$.

*E*

_{1}and

*E*

_{2}are two independent chance events such that

*E*

_{1}occurs with probability $1/2$ and

*E*

_{2}occurs with probability

*p*. And consider gambles

*G*

_{1}, ${G}_{1}^{-}$, and

*G*

_{2}whose outcomes depend on these two events:

${E}_{1}\&{E}_{2}$ | ${E}_{1}\&\neg {E}_{2}$ | $\neg {E}_{1}\&{E}_{2}$ | $\neg {E}_{1}\&\neg {E}_{2}$ | |
---|---|---|---|---|

$\left(1/2\right)\left(p\right)$ | $\left(1/2\right)(1-p)$ | $\left(1/2\right)\left(p\right)$ | $\left(1/2\right)(1-p)$ | |

G_{1} | A | D | B | C |

${G}_{1}^{-}$ | ${A}^{-}$ | ${D}^{-}$ | ${B}^{-}$ | ${C}^{-}$ |

G_{2} | ${B}^{--}$ | ${D}^{--}$ | ${A}^{--}$ | ${C}^{--}$ |

Like before, we assume that the Weak Principle of Equiprobable Unidimensional Dominance is a requirement of rationality. The Weak Principle of Equiprobable Unidimensional Dominance should be acceptable even if one is risk-averse.Footnote ^{126} In terms of risk, the dominated prospect must be less preferable than the dominating prospect. For every potential undesired final outcome of the dominating prospect, the dominated prospect has a corresponding (soured) final outcome with the same probability which is even less preferred. The probability of getting an undesired final outcome must be at least as high in the dominated prospect as in the dominating prospect. In any compelling violation of Independence for Constant Outcomes, no individual preference between two prospects violates the Weak Principle of Equiprobable Unidimensional Dominance. For instance, the Weak Principle of Equiprobable Unidimensional Dominance does not assume the point at issue against Allais and Ellsberg Preferences. None of the pairwise preferences in Allais and Ellsberg Preferences violates the Weak Principle of Equiprobable Unidimensional Dominance.

(41) ${G}_{1}\succ {G}_{1}^{-}\succ {G}_{2}$.

Now, consider the decision problem in Figure 13, the *Constant-Outcomes Money Pump*.Footnote ^{127}

At the initial choice node, you have a choice whether to accept a trade from *G*_{1} to
${G}_{1}^{-}$. If you turn down this trade, chance node 5 determines (depending on *E*_{1}) whether you will face node 6 or 9. If *E*_{1} occurs, you are offered a trade at node 6 from *ApD* to
$B{}^{--}p{D}^{--}$. And, if *E*_{1} does not occur, you are offered a trade at node 9 from *BpC* to
$A{}^{--}p{C}^{--}$.

At node 6, you would accept the trade from *ApD* to
$B{}^{--}p{D}^{--}$, since you prefer
$B{}^{--}p{D}^{--}$ to *ApD*. And, at node 9, you would also accept the trade from *BpC* to
$A{}^{--}p{C}^{--}$, since you prefer
$A{}^{--}p{C}^{--}$ to *BpC*.

Taking this into account at node 1, the choice at that node is effectively between
${G}_{1}^{-}$ (accepting the trade) and *G*_{2} (turning it down). Since you prefer
${G}_{1}^{-}$ to *G*_{2}, you accept the initial trade. So you end up with
${G}_{1}^{-}$ when you could have kept *G*_{1} for free. Moreover, note that
${G}_{1}^{-}$ is less preferred than *G*_{1} in every state of nature.

Hence we have a money-pump argument that preferences of the kind in (36) are irrational. Moreover, since the Constant-Outcomes Money Pump is BI-terminating, this argument need not assume that you retain your rationality at nodes that can’t be reached without making irrational choices.

Accordingly, we have a money-pump argument that the weak strict-preference version of Independence for Constant Outcomes is a requirement of rationality, and this argument relies on the following requirements of rationality:Backward induction at nodes that can be reached without making irrational choices

The Principle of Unexploitability

Unidimensional Continuity of Preference

The Weak Principle of Equiprobable Unidimensional Dominance

Decision-Tree Separability

The possibility of the Constant-Outcomes Money Pump

The Principle of Preferential Invulnerability

Since we can show that the weak strict-preference version of Independence for Constant Outcomes is a requirement of rationality and thereby that Allais and Ellsberg Preferences are irrational, we can rebut the most prominent objections to Independence.

#### 5.2 The weak strict-preference version of Independence

Having rebutted the most prominent objections to Independence (that is, the alleged rationality of Allais and Ellsberg Preferences), let us explore whether there are any compelling*positive*arguments that Independence is a requirement of rationality. We begin with the weakest version of Independence, namely,

*Independence (the weak strict-preference version)* For all probabilities *p* such that
$0<p<1$, it holds that, if
$X\succ Y$, then it is not the case that
$YpZ\succ XpZ$.

This version of Independence can be shown to be a requirement of rationality with the help of a money-pump argument with even weaker assumptions than those we relied on for the argument that Independence for Constant Outcomes is a requirement of rationality.

Violations of the weak strict-preference version can only be of the following kind, where*p*is a probability such that $0<p<1$:

(42) $A\succ B$, and $BpC\succ ApC$.

(43) $BpC\succ {B}^{-}p{C}^{-}\succ ApC$,

where
${B}^{-}p{C}^{-}$ is a souring of *BpC*.

Now, consider the decision problem in Figure 14, the *Independence Money Pump*.Footnote ^{128}

Here, the two chance nodes depend on the same event *E*, which occurs with probability *p*. You start off with *BpC*. At node 1, you are offered a trade from *BpC* to
${B}^{-}p{C}^{-}$. If you accept this trade, then, if *E* occurs, you end up with
${B}^{-}$ and, if *E* does not occur, you end up with
${C}^{-}$. If you turn the trade down and *E* occurs, you will be offered a trade from *B* to *A* at node 4. And, if you turn down the trade at node 1 and *E* does not occur, you end up with *C*.

Since you prefer *A* to *B*, you would accept the trade at node 4. Using backward induction at node 1, the prospect of going down is then effectively *ApC* and the prospect of going up is
${B}^{-}p{C}^{-}$. So you go up at node 1, since you prefer
${B}^{-}p{C}^{-}$ to *ApC*. But then you end up with
${B}^{-}p{C}^{-}$ when you could have kept *BpC* for free if you had followed the plan to go down at each choice node. And, since the chance nodes depend on the same event, we find that, in every state of nature, the prospect of going up at node 1 is a souring of the prospect of following the plan to go down at each choice node.

Backward induction at nodes that can be reached without making irrational choices

The Principle of Unexploitability

Unidimensional Continuity of Preference

Decision-Tree Separability

The possibility of the Independence Money Pump

The Principle of Preferential Invulnerability

Still, axiomatizations of Expected Utility Theory typically need a stronger version of Independence, like the strong strict-preference version.

As mentioned earlier, Expected Utility Theory can be axiomatized by Completeness, Transitivity, Continuity, and the strong strict-preference version of Independence. Can we strengthen this standard axiomatization so that it relies on the *weak* strict-preference version of Independence rather than the strong one? We cannot. Likewise, we cannot replace the strong strict-preference version of Independence with the weak strict-preference version of Independence for Constant Outcomes in the axiomatization.Footnote ^{129}

#### 5.3 The strong strict-preference version of Independence

So let us turn to*Independence (the strong strict-preference version)* For all probabilities *p* such that
$0<p<1$, it holds that, if
$X\succ Y$, then
$XpZ\succ YpZ$.

(42) $A\succ B$, and $BpC\succ ApC$.

*p*is a probability such that $0<p<1$:

(44) $A\succ B$, and $ApC\sim BpC$.

(45) $A\succ B$, and $ApC\parallel BpC$.

The money-pump argument in Section 5.2 doesn’t work against the preferences in (44) and (45). Preferences of the kind in (45) are ruled out by Completeness. The preferences in (44) are more challenging. These preferences violate the strong strict-preference version of Independence, but they do not violate the other standard axioms of Expected Utility Theory.Footnote ^{130} And, since the biconditional weak-preference version of Independence is logically stronger than the strong strict-preference version, the preferences in (44) violate that version too. Hence, to have a cogent argument that these versions of Independence are requirements of rationality, we must show that the preferences of the kind in (44) are irrational.

*The Strong Principle of Unidimensional Stochastic Dominance* If (i)
${X}^{-}$ is a souring of *X*, (ii)
$X\succ {X}^{-}$, and (iii)
$0<p<1$, then
$XpY\succ {X}^{-}pY$.

Just like the Weak Principle of Equiprobable Unidimensional Dominance, this requirement should be acceptable even if one is risk-averse. The probability of getting an undesired final outcome must be at least as high in the dominated prospect as in the dominating prospect.Footnote ^{131} In any compelling violation of Independence, the individual pairwise preferences do not violate the Strong Principle of Unidimensional Stochastic Dominance.

We will show that preferences of the kind in (42) can be derived from (44) – given that the Strong Principle of Unidimensional Stochastic Dominance, Transitivity, and Unidimensional Continuity of Preference are requirements of rationality.

From (44), we have, by Unidimensional Continuity of Preference,(46) $A\succ {A}^{-}\succ B$,

*A*. From (46), we have, by the Strong Principle of Unidimensional Stochastic Dominance,

(47) $ApC\succ {A}^{-}pC$.

(48) $BpC\succ {A}^{-}pC$.

(49) ${A}^{-}\succ B$, and $BpC\succ {A}^{-}pC$.

Hence, from (44), we have derived preferences of the kind in (42). Since preferences of that kind can be shown to be irrational by the money-pump argument for the weak strict-preferences version (Section 5.2), we can show that preferences of the kind in (44) are irrational.

Accordingly, we have a money-pump argument that the strong strict-preference version of Independence is a requirement of rationality, and this argument relies on the following requirements of rationality:Backward induction at nodes that can be reached without making irrational choices

Completeness (defended in Section 3)

The Principle of Unexploitability

The Strong Principle of Unidimensional Stochastic Dominance

Transitivity (defended in Section 4.1)

Unidimensional Continuity of Preference

Decision-Tree Separability

The possibility of the Independence Money Pump

The Principle of Preferential Invulnerability

Note that this argument requires stronger assumptions than those needed in the argument for the weak strict-preference version of Independence, since that argument did not need Completeness, the Strong Principle of Unidimensional Stochastic Dominance, and Transitivity.

#### 5.4 The biconditional weak-preference version of Independence

Finally, let us turn to*Independence (the biconditional weak-preference version)* For all probabilities *p* such that
$0<p<1$, it holds that
$X\succsim Y$ if and only if
$XpZ\succsim YpZ$.

With only slightly stronger assumptions than those for the argument that the strong strict-preference version is a requirement of rationality, we can show that the biconditional weak-preference version is a requirement of rationality.

In addition to preferences of the kind in (42), (44), and (45) which we have already shown are irrational (with the arguments in Sections 5.2 and 5.3), violations of the biconditional weak-preference version of Independence can also be of the following kinds, where again*p*is a probability such that $0<p<1$:

(50) $A\sim B$, and $ApC\succ BpC$.

(51) $A\sim B$, and $ApC\parallel BpC$.

(52) $A\parallel B$, and $ApC\succsim BpC$.

Two of these violations – namely, (51) and (52) – are ruled out by Completeness. So, to finish the argument that the biconditional weak-preference version is a requirement of rationality, we need only show that preferences of kind in (50) are irrational.

From (50), we have, by Unidimensional Continuity of Preference,(53) $ApC\succ {A}^{-}p{C}^{-}\succ BpC$,

*ApC*. Since ${A}^{-}p{C}^{-}$ is a souring of

*ApC*, it follows that ${A}^{-}$ is a souring of

*A*and that ${C}^{-}$ is a souring of

*C*. So we have, by the Souring Principle,

(54) $A\succ {A}^{-}$, and $C\succ {C}^{-}$.

(55) $B\succ {A}^{-}$.

(56) ${A}^{-}pC\succ {A}^{-}p{C}^{-}$.

(57) ${A}^{-}pC\succ BpC$.

(58) $B\succ {A}^{-}$, and ${A}^{-}pC\succ BpC$.

We have, once more, derived preferences of the same kind as those in (42). And, since such preferences can be shown to be irrational by the money-pump argument in Section 5.2, we can show that preferences of the kind in (50) are irrational.

Hence we have a money-pump argument that the biconditional weak-preference version of Independence is a requirement of rationality, and this argument relies on the following requirements of rationality:Footnote^{132}

Backward induction at nodes that can be reached without making irrational choices

Completeness (defended in Section 3)

The Principle of Unexploitability

The Souring Principle

The Strong Principle of Unidimensional Stochastic Dominance

Transitivity (defended in Section 4.1)

Unidimensional Continuity of Preference

Decision-Tree Separability

The possibility of the Independence Money Pump

The Principle of Preferential Invulnerability

Note that, apart from the addition of the Souring Principle, this argument for the biconditional weak-preference version does not require stronger assumptions than the argument for the strong strict-preference version in Section 5.3.

### 6 Continuity

Suppose that you prefer having two candy bars to having one candy bar and that you prefer either of these alternatives to suddenly dying. Yet you’re not willing to risk any chance of sudden death for a chance of having two candy bars rather than just one.Footnote ^{133}

^{134}

*Continuity* If
$X\succ Y\succ Z$, then there are probabilities *p* and *q* such that (i)
$0<p<1$, (ii)
$0<q<1$, and (iii)
$XpZ\succ Y\succ XqZ$.

(59) $A\succ B\succ C$, and $ApC\succ B$ for all probabilities

*p*such that $0<p<1$.(60) $A\succ B\succ C$, and $B\succ ApC$ for all probabilities

*p*such that $0<p<1$.(61) $A\succ B\succ C$, and $B\sim ApC$ for some probability

*p*such that $0<p<1$, and(i) there is no probability

*q*such that $0<q<1$ and $AqC\succ B$ or(ii) there is no probability

*r*such that $0<r<1$ and $B\succ ArC$.

(62) $A\succ B\succ C$, and $B\parallel ApC$ for some probability

*p*such that $0<p<1$, and(i) there is no probability

*q*such that $0<q<1$ and $AqC\succ B$ or(ii) there is no probability

*r*such that $0<r<1$ and $B\succ ArC$.

(63) $A\succ B\succ C$, and $B\sim ApC$.

(64) $A\succ C$.

*q*and

*r*be probabilities such that $0<r<p<q<1$. Then, from (64), we have, by the strong strict-preference version of Independence,

(65) $AqC\succ ApC\succ ArC$.

(66) $AqC\succ B\succ ArC$.

And (66) rules out (61), since $0<r<q<1$.

So, to complete the argument that Continuity is a requirement of rationality, we must show that the remaining kinds of violations are irrational. That is, we need to show that preferences of the kind in (59) and (60) are irrational.

Suppose that you violate Continuity by having the preferences in (59). From (59), we have, by Unidimensional Continuity of Dispreference,(67) $B\succ {C}^{+}\succ C$,

*C*such that ${C}^{+}$ is certainly

*ε*units superior to

*C*in a dimension you care about. From (59) and (67), we have, by Transitivity,

(68) $ApC\succ {C}^{+}\succ C$ for all probabilities

*p*such that $0<p<1$.

*A*’, ‘

*C*’, and ‘ ${C}^{+}$’ now be ‘ ${A}^{-}$’, ‘ ${C}^{-}$’, and ‘

*C*’ respectively. Given this relabelling, (68) becomes

(69) ${A}^{-}p{C}^{-}\succ C\succ {C}^{-}$ for all probabilities

*p*such that $0<p<1$.

We now let *A* be a sweetening of
${A}^{-}$ such that *A* is certainly *ε* units superior to
${A}^{-}$ in the dimension that *C* and
${C}^{-}$ differ. Consider the decision problem in Figure 15, the *Lexi-Optimist Pump*.Footnote ^{135}

*q*gets to 0 (without reaching 0), you still pay the fixed positive amount

*ε*to get

*AqC*. So, starting off with

*C*, you are still willing to pay a fixed – not arbitrarily small – amount

*ε*to trade

*C*for

*ApC*, which is arbitrarily similar to

*C*(that is, arbitrarily likely to result in the same final outcome as

*C*). So an exploiter can get a payment of

*ε*from you with only an arbitrarily small chance of having to give you anything – that is, the arbitrarily small chance of having to trade you

*A*for

*C*. This is arbitrarily close to pure exploitation. You violate the following requirement:

*The Principle of Limit Unexploitability* If (i) *ε* is a fixed positive amount, (ii)
${X}^{-}$ is a souring of *X* such that
${X}^{-}$ is certainly *ε* units inferior to *X* in a dimension one cares about, (iii)
$X\succ {X}^{-}$, (iv), at node *n*, *P* and
${P}^{-}$ are two available plans such that *P* is the only available plan that amounts to walking away from all offers by an exploiter and the prospect of following *P* is *X* and the prospect of following
${P}^{-}$ is arbitrarily likely to be
${X}^{-}$, and (v) one knows what decision problem one faces at *n*, then one does not follow
${P}^{-}$ from *n*.

Is it as plausible that the Principle of Limit Unexploitability is a requirement of rationality as that the Principle of Unexploitability is one? Maybe not, but the former is still compelling as a requirement of rationality.Footnote ^{136} If you violate the Principle of Limit Unexploitability, an exploiter can get a fixed amount of money from you for the arbitrarily small cost of an arbitrarily small chance of having to trade you something (in this case, the chance of having to trade you *A* for *C*).

It may be objected that, if you prefer *A* to *C* by an infinite amount, then it’s not a clear sign of irrationality to choose
${A}^{-}q{C}^{-}$ rather than *C* with *q* arbitrarily close to 0. Yet, if you prefer *A* to *C* by an infinite amount, you should presumably be willing to pay not only a small amount but any finite amount to get *AqC* rather than *C*. So you would pay an arbitrarily large amount to increase the chance of getting *A* rather than *C* by an arbitrarily small amount, which seems fanatic.Footnote ^{137}

(70) $A\succ {A}^{-}\succ B$,

*A*which is certainly

*ε*units inferior in a dimension you care about. From (60) and (70), we have, by Transitivity,

(71) $A\succ {A}^{-}\succ ApC$ for all probabilities

*p*such that $0<p<1$.

Finally, consider the decision problem in Figure 16, the *Lexi-Pessimist Pump*.

In this case, no matter how close *q* gets to 1 (without reaching 1), you still pay the fixed amount *ε* to get *A*. So, starting off with *AqC*, which is arbitrarily likely to result in the same final outcome as *A*, you are still willing to pay a fixed positive amount *ε* to get *A* instead. So you violate the Principle of Limit Unexploitability.

Completeness (defended in Section 3)

The Principle of Limit Unexploitability

The strong strict-preference version of Independence (defended in Section 5.3)

Transitivity (defended in Section 4.1)

Unidimensional Continuity of Dispreference

Unidimensional Continuity of Preference

The possibility of the Lexi-Optimist Pump

The possibility of the Lexi-Pessimist Pump

The Principle of Preferential Invulnerability

This argument – when added to the arguments in Sections 3, 4.1, and 5.3 – completes the overall argument that rational preferences conform to Expected Utility Theory.

### 7 Against Resolute Choice

A common objection to money-pump arguments is that they don’t work against agents who follow resolute choice. *Resolute choice* is the approach of choosing in accordance with the plans one has adopted even if one wouldn’t choose in accordance with those plans if one hadn’t adopted them.Footnote ^{138} In the Upfront Money Pump, for instance, a resolute agent with the cyclic preferences in (1) could stick to the plan of turning down all trades so that they end up with *A* rather than
${A}^{-}$. And then the money-pump argument is blocked.

While resolute choice may seem like a plausible response to money-pump arguments, that plausibility evaporates once it is spelled out how the approach is supposed to work. So how, more precisely, is the resolute-choice approach supposed to work? In the literature, there are six separate ways to be resolute: the Counter-Preferential Approach, the Revision Approach, the Constraint Approach, the Second-Order Approach, the Fine-Grained Approach, and the Conservative Approach. As we shall see, these approaches are all implausible as responses to money-pump arguments.

First, consider the Counter-Preferential Approach:Footnote^{139}

*The Counter-Preferential Approach* When you adopt a plan, you follow that plan even if you prefer not to follow it.

On this approach, if you adopt the plan to stick with *A* in the Upfront Money Pump, you will follow this plan even though you prefer to deviate at node 3. And then you avoid exploitation.

The problem with this approach is that choosing against your own preference at the moment of choice is irrational.Footnote ^{140} The descriptions of the final outcomes should capture everything you care about. So your preferences over these final outcomes and prospects should capture everything you care about.Footnote ^{141} And, if you all-things-considered prefer to deviate from the plan, it would be irrational to follow the plan.

It may be objected that there is still an instrumental rationale for following a plan even though you prefer to deviate – namely, the rationale that the prospect of following the plan at the later node is preferred to what was, at the node the plan was adopted, the prospect of not adopting the plan.Footnote ^{142} But this rationale (which may be compelling at the node you adopt the plan) is no longer compelling at the later node where you prefer to deviate. Because, once you prefer to deviate from the plan, the plan no longer achieves your ends. The alternative to the prospect of following the plan is no longer the prospect you would have faced had you not adopted the plan; the alternative is a prospect you prefer to the prospect of following the plan. So this instrumental rationale for the Counter-Preferential Approach is implausible.Footnote ^{143}

^{144}

*The Revision Approach* When you adopt a plan, you revise your preferences so that you prefer to act in accordance with the plan at all future choice nodes (even though you would have preferred to deviate from the plan if you had not adopted it).

Once you adopt the plan to turn down all offers in the Upfront Money Pump following the Revision Approach, you prefer *A* to each of
${A}^{-}$, *B*, and *C*. Hence you would no longer be tempted to accept any of the offers. And then you end up with *A* and avoid exploitation.

The Revision Approach has a significant drawback as a defence against money-pump *arguments* (as opposed to money pumps): If agents with a certain set of preferences have to adopt some other set of preferences to escape exploitation, then the original preferences still seem irrational. So, in this case, the defeat of money pumps is a victory for money-pump arguments.Footnote ^{145}

Moreover, following the Revision Approach, there are at least two separate ways of turning down the first offer in the Upfront Money Pump. You could turn down the first offer without adopting the plan to turn down all offers (without revising your preferences) and, alternatively, you could turn down the first offer by adopting that plan (and thereby revise your preferences). But, if the latter option is available at the first node, it should be added to the decision tree. (The decision tree should reflect all your choices in the decision problem.) This shows that the Revision Approach does not apply to the original decision problem where this option is unavailable.

Furthermore, if we include the extra option of adopting the plan to turn down all trades and thereby revise your preferences, the prospect of that option need not be the same as the prospect of turning down all trades without having revised your preferences. So we may include this extra option yet make it unattractive (that is, unattractive before you revise your preferences) by imposing a cost to revising your preferences. And then, since this option would be less preferred than the other options given that the cost is sufficiently high, it would be irrational to choose this extra option. And, once this extra option is ruled out, you effectively face the original exploitation scheme.

It may be objected that there are many prospects you haven’t considered and you only form your view about them once you face a choice between them. So it may seem rationally permitted to have a preferential gap between some prospects and then revise this gap to a strict preference or indifference once you face a choice between them. This objection, however, relies on an implausible understanding of preferential gaps. The existence of prospects that you still haven’t compared does not entail that you have a preferential gap between those prospects. As sketched earlier (at the end of Section 3), I take a preference relation to be a disposition to have a psychologically real mental ranking of the prospects if you were to compare them. You don’t need to have a preferential gap between two prospects when you merely haven’t got around to comparing them. You have a preferential gap between two prospects in case neither prospect would rank at least as high as the other in your mental ranking if you were to compare them. If preferential gaps in this sense need to be revised in order to avoid money pumps, such gaps are irrational.

Now, consider the Constraint Approach:Footnote^{146}

*The Constraint Approach* When you adopt a plan, you are no longer able to deviate from the plan at future choice nodes.

If you follow the Constraint Approach and adopt the plan to turn down all offers in the Upfront Money Pump, you’re no longer able to accept the offers at nodes 2 and 3. So you would end up with *A* and avoid exploitation.

Nevertheless, the Constraint Approach requires an implausible account of ability.Footnote ^{147} It seems that, after you have adopted a plan, you can still deviate from the plan. Even if you will in fact end up sticking with the plan at later choice nodes, you still have the ability to deviate at those nodes. But this picture, which is suggested by the phenomenology of planning, conflicts with the Constraint Approach.

Furthermore, like the Revision Approach, the Constraint Approach conflicts with the specifications of the decision problems for the money-pump arguments. Consider, for instance, the Upfront Money Pump. In the original decision problem, you do have the opportunity to accept the offers at nodes 2 and 3. So to adopt the plan to turn down all offers in a way that removes those later opportunities would be an additional option at the initial node.Footnote ^{148} But then the Constraint Approach doesn’t apply to the original decision problem where this option is unavailable at the initial node. So the Constraint Approach does not help you escape being money pumped in the original decision problem.Footnote ^{149}

And, as before, we also note that, if we include the extra option of constraining yourself to turning down all trades, the prospect of that option need not be the same as the prospect of turning down all trades without having constrained yourself. So we may include this extra option yet make it unattractive by imposing a cost to constraining yourself. Since this option would be less preferred than the other options (given that the cost is sufficiently high), it would be irrational to choose this extra option. And then, once this extra option is ruled out, you effectively face the original exploitation scheme.

Let us turn to the Second-Order Approach:Footnote^{150}

*The Second-Order Approach* When you adopt a plan, your first-order preferences remain the same but you have a second-order preference for steadfastness which motivates choosing to act in accordance with the plan at all future nodes.

You can follow the Second-Order Approach by having a second-order preference for not being exploited which trumps your other preferences. So, even though you prefer
${A}^{-}$ to *B* in the Standard Money Pump, you would turn down the offer to trade from *B* to
${A}^{-}$ at node 3. What counts against
${A}^{-}$ at node 3 is that you could have had *A* (had you chosen otherwise earlier). So, even though you prefer
${A}^{-}$ to *B*, you prefer *B* to
${A}^{-}$-*when-you-could-have-had*-*A*.

A problem for the Second-Order Approach is that any second-order preference for honouring previous commitments should be included in your overall, first-order preferences, which are what the axioms of Expected Utility Theory apply to.Footnote ^{151} And then, given that your overall, first-order preferences concern more complex options that include previous commitments, there’s no need to introduce any second-order preferences.

^{152}

*The Fine-Grained Approach* When you adopt a plan, you prefer to stick to that plan and, since you care about whether you stick to the plan, the final outcomes include information about whether they were reached by sticking to the plan.

More generally, you care not only about what your final holding will be but also about how you arrive at that final holding relative to the whole decision tree. So your first-order preferences are over prospects where the final outcomes include information about what could have been chosen in the decision tree.Footnote ^{153}

Once we have made sure that your preferences cover prospects with final outcomes that cover everything you care about, it may be that the decision problems we have relied on for the money-pump arguments are no longer possible. Because the final outcomes would include information about what the rest of the decision tree looks like, that information restricts what decision trees they could be part of. Likewise, once we transform the final outcomes in a decision problem so that they include the information about the rest of the decision problem, the agent need no longer have the preferences we have assumed over their options at the choice nodes.

Nevertheless, the Fine-Grained Approach conflicts with some plausible restrictions on what it is rationally permitted to prefer. Suppose that, in the Upfront Money Pump, you adopted at node 1 the plan to stick with *A* (that is, you plan to turn down all trade offers). At node 3, you have a choice between *A* and *C*. Yet, given your fine-grained preference for sticking to your plan, we should represent these options as *A*-*and-sticking-to-the-plan* and *C*-*and-deviating-from-the-plan*. Even though you prefer *C* to *A*, you prefer *A*-*and-sticking-to-the-plan* to *C*-*and-deviating-from-the-plan*. But why would you care at node 3 whether you stick to the plan you adopted at node 1? Whatever reasons you had for adopting that plan are no longer relevant. The bare fact that you decided to adopt the plan in the past does not provide a reason for sticking to the plan.Footnote ^{154} And, once the fact that you adopted the plan is left out, not only do you now prefer that you choose *C* over *A* but you also preferred, at the time you adopted the plan, that you would choose *C* over *A*. So caring about this plan for its own sake seems irrational. (This substantial restriction on what can be rationally preferred is, of course, a departure from a purely formal approach to decision theory that imposes no such restrictions.Footnote ^{155})

Next, suppose that you have an overriding preference not to be money pumped.Footnote ^{156} Accordingly, in any choice between two options where you are money pumped in one option but not in the other, you prefer not to be money pumped. But caring about being money pumped for its own sake seems irrational. Consider, for instance, the choice at node 3 in the Standard Money Pump. At this node, you have a choice between
${A}^{-}$ and *B*. Given your fine-grained preference for not being money pumped, we should represent these options as
${A}^{-}$-*and-being-money-pumped* and *B*-*and-not-being-money-pumped*. Even though you prefer
${A}^{-}$ to *B*, you prefer *B*-*and-not-being-money-pumped* to
${A}^{-}$-*and-being-money-pumped*. Choosing
${A}^{-}$ at node 3 amounts to being money pumped because you could have kept *A* for free if you had turned down the trade at node 1. But, at node 3, why would you care whether you could have had *A* rather than
${A}^{-}$?Footnote ^{157} The loss of the opportunity to have *A* for free is a sunk cost at that point.Footnote ^{158} To prefer, for its own sake, that one isn’t money pumped seems irrational.

(Does this claim rule out the irrationality of being money pumped? It does not. It is irrational to pay for something you could keep for free given that you prefer having more money other things being equal, but it needn’t be irrational to pay for something you can no longer have for free.)

It may be objected that, in the Upfront Money Pump (and other BI-terminating money pumps), the Fine-Grained Approach need not attach any significance to sunken costs. To block exploitation, the fine-grained preference only needs to make you turn down the first trade. There is no sunk cost when you consider the first trade, since you are still at the initial node.

But consider the choice nodes you could face if you were to turn down the trade at the initial node, that is, nodes 2 and 3. At those nodes, you can no longer be money pumped. So, for the feasible prospects that remain at those nodes, your fine-grained preferences should be the same as your course-grained preferences (that is, your preferences over prospects where the final outcomes do not include information about what could have been chosen in the decision tree). So, by backward induction, we find that you would accept the trade at node 3 and therefore that you would accept the trade at node 2. Taking this into account at node 1, you see that you wouldn’t end up with *A* if you turned down the initial trade. So the choice at node 1 is effectively between
${A}^{-}$ and *B*. Given your fine-grained preference for not being money pumped, we should represent these options as
${A}^{-}$-*and-being-money-pumped* and *B*-*and-not-being-money-pumped*. So, while you prefer
${A}^{-}$ to *B*, you prefer *B*-*and-not-being-money-pumped* to
${A}^{-}$-*and-being-money-pumped*. But, once more, it seems irrational to care, for its own sake, about being money pumped. The problem with being money pumped by paying for *A* is that you could have kept *A* for free. But, given your allegedly rational preferences, you predict that you won’t keep *A*. So, since you know at node 1 that you cannot rationally end up with *A*, you should regard giving up *A* as a cost that will be sunk. Hence, even in the Upfront Money Pump (and other BI-terminating money pumps), your fine-grained preference for not being money-pumped seems irrational.

^{159}

*The Conservative Approach* When you adopt a plan, you follow that plan as long as you do not prefer not to follow it.

This approach does not help against the Upfront Money Pump, but, as a requirement of rationality, it will block the argument for Minimal Unidimensional Precaution and the arguments that rely on precautionary or dominance backward induction. So it would block the money-pump argument for Completeness which is based on the Precaution Money Pump.

The Conservative Approach does not require that you choose against your preference. So the objection we raised to the Counter-Preferential Approach does not apply here.

But, if you don’t prefer following the plan to deviating from it, then it’s hard to see what would be irrational about choosing to deviate. Consider, for instance, the choice at node 4 in the Precaution Money Pump, where you have a choice between *A* and
${B}^{-}$. And suppose that, at the initial node, you adopted the plan to walk away with *A*. At node 4, you still know that, at the time you adopted the plan, you did not prefer that plan to the one that ends with choosing
${B}^{-}$. The only reason you didn’t adopt the plan that ends with choosing
${B}^{-}$ was presumably that it is less preferred than the (initially available) plan that ends with choosing *B*. But this reason no longer applies at node 4 where the latter plan is unavailable. So it does not seem irrational, at node 4, to deviate from the adopted plan and choose
${B}^{-}$