The psychologist's problem. Since its inception in the 19th century, psychological science has made steady progress investigating perceptual and motoric abilities—how the visual system encodes color, for example, or how we shift our gaze to peripheral events. Much less is understood about abilities thatmake us distinctly human. Some information is available about themechanisms of natural language. But there is hardly any insight into how people create scientific theories about the world. This is an embarassing gap for Psychology since scientific achievement is themost distinctive and remarkable feature of our species.

What's blocking progress is that the most natural account of this ability seems to face an insuperable difficulty. According to the account in question, most everyone has an innate disposition to reason in rough conformity with normatively correct principles of deductive and inductive logic—just as most everyone is endowed with perceptual mechanisms that give us a roughly accurate picture of the environment. How else could our ancestors have met the challenges of survival? It is the twin pillars of natural reasoning—deductive and inductive—that allow people to draw out the consequences of rival scientific theories and assign sensible credibilities to each in the light of data.

These vague remarks are just an attempt to prepare for sharper theories. But we stumble even at this initial step, because one of the twin pillars seems to be absent. The problem is not so much with deductive logic. It can be challenging to communicate the informal concept of logical necessity. But once this is achieved, most people distinguish validity from invalidity on an intuitive basis across a broad class of arguments.

§1. Introduction. Logical calculi were invented to model mathematical thinking and to formalize mathematical arguments. The calculi of Boole [8] and of Frege [15] can be considered as the first mathematical models of logical inference. Their work paved the way for the discipline of metamathematics, where mathematical reasoning itself is the object of mathematical investigation. The early calculi, the so-called Hilbert type- and Gentzen-type calculi [25], [17] developed in the 20th century served the main purpose to analyze and to reconstruct mathematical proofs and to investigate provability. A practical use of these calculi, i.e., using them for solving actual problems (e.g., for proving theorems in “real” mathematics), was not intended and even did not make sense.

But the idea of a logical calculus as a problem solver is in fact much older than the origin of propositional and predicate logic in the 19th century. Indeed this idea can be traced back toG.W. Leibniz with his brave vision of a calculusratiocinator [29], a calculus which would allow solution of arbitrary problems by purely mechanical computation, once they have been represented in a special formalism. Today we know that, even for restricted languages, this dream of a complete mechanization is not realizable—not even in principle (we just refer to the famous results of Gödel [20] and Turing [39]). That does not imply that we have to reject the idea altogether. Still it makes sense to search for a lean version of the calculus ratiocinator. Concerning the logical language, the ideal candidate is first-order logic; it is axiomatizable (and thus semidecidable), well-understood and sufficiently expressive to represent relevant mathematical structures. By Church's result [10] we know that there is no decision procedure for the validity problem of first-order logic; thus there is no procedure which is 1. capable of verifying the validity of all valid formulas and 2. terminating on all formulas. So, even in first-order logic, we have to be content with the verification of problems. The only thing we can hope for is a calculus which offers a basis for efficient proof search. It is not surprising that the invention of the computer lead to a revival of Leibniz's dream.

In Chapters 6 through 9, we surveyed the personal basic belief-generating mechanisms. These mechanisms form beliefs upon the basis either of how we are appeared to in various ways or of connections we intuit. The mechanism in this chapter, by contrast, takes beliefs ready formed by someone else and makes them our own. This is a central belief-generating mechanism for argumentation. In accepting the basic premises of an argument, in many cases, one may be taking someone's word for it, either the proponent's or someone else's. Let us begin by reflecting on just how central is taking one's word.

IMPORTANCE OF TAKING ONE'S WORD

Certain recent work in epistemology has called attention to two facts: the significant extent to which in forming our beliefs we rely on the word of others and the meager extent to which philosophers have considered this mode of forming beliefs. Simple reflection should reveal the extent to which our beliefs are formed by taking someone else's word. Coady makes this point with great persuasiveness (1992, pp. 6–7). He invites you to suppose you are visiting Amsterdam for the first time. You rely on the word of the airplane's crew that this really is Amsterdam where your plane had landed. You enter your date and place of birth on the hotel registration form. The belief that a certain date is one's birthday or that a certain place is one's place of birth is always received from others.

In Chapter 5, we characterized interpretations as contingent nonevaluative intensional statements and developed their connection with explanations. An interpretation typically seeks to render events or conditions meaningful. This is done by somehow relating events nomically or indicating how some event or some condition is to be explained and thus related nomically to certain explanatory factors. In this chapter, we are attempting to determine, first, whether any interpretations are basic beliefs and, second, whether there is a presumption of reliability for the mechanism or mechanisms generating those basic interpretive beliefs. The connection between interpretations and explanations yields a strategy for proceeding. First, what sorts of explanations are presupposed by interpretations? Second, is there some core concept common to these explanations in terms of which the various types of interpretations can be explicated? Assuming this second question can be answered positively – and we shall argue that it can – we may then properly inquire about what mechanisms generate basic beliefs involving this core concept.

THREE TYPES OF EXPLANATIONS

As in Chapter 7 we distinguished physical, personal, and institutional perception, and correspondingly physical, personal, and institutional facts, so we may distinguish physical, personal, and institutional explanations. Physical explanations are a subclass of causal explanations. Some event or phenomenon in the physical world is explained in terms of some antecedent physical event or phenomenon and a covering generalization.

1. The match lit because it was struck.

This explanation explicitly appeals to the antecedent striking of the match to explain its lighting.

When, if ever, is a premise – indeed a statement in general – acceptable? That is the central question of this book. Therefore, this is a normative investigation. This point needs to be underlined, as the very word “acceptability” contains an ambiguity. A statement's acceptability may mean its prospects for being accepted by a certain audience. This is not our meaning. We are not interested in the marketability of a statement but in whether the statement should be accepted. Is acceptance rationally justified for a particular audience? However, there are two preliminary issues we must address. Why is this book needed at all? Is there no simple, straightforward, and adequate answer available? The simplest way to address this question is to look at certain simple and straightforward answers and see that they either do not answer the question correctly or are fraught with problems. But first we should clarify what it means to accept a statement, and so by implication what “acceptability” means.

ACCEPTANCE – A BASIC DEFINITION

In (1992), L. Jonathan Cohen contrasts these two concepts: To believe a proposition that p is to be disposed to feel that p is true and that not-p is false, whether or not one is prepared to take that p as a premise for further belief or action. To accept that p is to take that p as a premise “for deciding what to do or think in a particular context, whether or not one feels it to be true that p” (1992, p. 4).

Some premises are straightforwardly acceptable as basic premises without argument either presenting evidence for the premises or a case for their being justified in this instance. For example, given how a challenger is appeared to perceptually, she may be perfectly justified in her immediate acceptance of a premise expressing a perceptually generated descriptive belief. However, suppose one is faced with a “hard” case. One may feel constrained to justify to oneself that a statement is acceptable as a basic premise. Alternatively, one may need to show to someone else why a statement is acceptable – or not acceptable – as a basic premise under prevailing conditions. Here the request is to justify the judgment that a particular statement is or is not acceptable as a basic premise. Apart from the dialectical requirement of showing someone that a statement is acceptable, the epistemological issue remains of why one is justified in accepting a particular premise in a certain situation.

At the risk of raising hackles in some, we call making such a determination an exercise in epistemic casuistry. There is no need for hackles to be raised, given the way we use the word. For “casuistry” as we understand it means no more than what we have indicated – making determinations in particular cases. In its ordinary, moral context, casuistry deals with such questions as whether a particular act is right or wrong, or a particular state of affairs good or bad.

Our goal in this chapter is first to identify the various personal mechanisms that generate basic beliefs that are descriptions and then to discuss whether and under what circumstances each of these mechanisms is presumptively reliable, and what grounds that presumption. Our first step again will be to determine what types of descriptions there are. For each type, we proceed to ask what personal mechanisms generate basic descriptive beliefs of that type. This is our project in the next section. In subsequent sections we shall discuss the presumptive reliability of each of the mechanisms identified.

WHAT TYPES OF DESCRIPTIONS ARE THERE?

Let us begin by looking at some intuitive examples of descriptions:

1. A bus is passing my office window.

2. The house across the street is painted white.

3. I have a pain in my right leg.

4. The last time my friends were over for dinner I served chicken.

5. I felt a curious sensation of warmth when the medication was injected.

6. The program will come on in a half-hour.

7. The sun will rise tomorrow.

8. During the second week of January, thirty homeless persons were found dead on the streets of New York.

9. Fifty percent of the voters polled said they disapproved of the president's job performance.

10. Fifty percent of the voting population disapproves of the president's job performance.

11. All swans are white.

12. None of the children in the room have brought their lunches.

These examples illustrate the various types of descriptions. The distinction of reports from generalizations dividing statements (1)–(8) from (9)–(12) is principal.

We have given a definition of challenger presumption and indicated how challenger presumption may serve as a criterion of acceptability. As we noted at the end of the last chapter, if this is to advance our ability to determine when a statement is acceptable, we must be able to determine under what circumstances there is a presumption for a statement from a challenger's point of view. As Rescher points out in (1988), “The rational legitimation of a presumptively justified belief lies in the fact that some ‘suitably favorable indication’ speaks on its behalf, and no already justified counter-indication speaks against it” (1988, p. 50). Our task now is to identify these suitably favorable indicators. Our first step will be to look at principles of presumption that have actually been advanced and accepted. We have already noted two, a presumption in favor of the senses and in favor of memory.

Before proceeding to our basic survey of the principles of presumption, we must develop a point implicit in the preceding paragraph. Rescher speaks of the rational legitimation of a presumptively justified belief. This suggests a distinction between presumptively justified beliefs, and establishing that there is a presumption for such beliefs. If there is a presumption for a belief for a particular person qua challenger, need she be aware of that presumption?