DECISION PRINCIPLE

Decision theory addresses two questions. What states of mind or sets of judgements are rationally permissible? And what choices should we make, given our state of mind or judgement? The previous chapters have proposed an answer to the first question and enough is now in place for us to turn to the second.

There is broad agreement amongst decision theorists that, under the conditions characterised by subjective uncertainty about the state of the world and in which the agent has determinate degrees of belief in, and preference for, all relevant prospects, actions should be chosen in accordance with the expected subjective benefit of so doing. In contrast, there is a good deal of variety in the way that this idea is cashed out. Much of this is driven by differences in the way that decision problems, and especially the objects of choice, are conceived and formally represented. But some of it emanates from more substantial differences both in the interpretation of the principle of maximising expected benefit and in the formal assumptions made for the purposes of deriving it. One aim of this chapter will be to explore the relationship between these theories. Another will be to defend a particular one. As the theory of rationality presented in the previous chapter is essentially an extension of Richard Jeffrey's decision theory, it is natural to start by reminding ourselves about what he says about choice of action. In Jeffrey's theory, an action is simply a prospect that can be made true at will (i.e. which is a genuine option for an agent to exercise) and its choice-worthiness is identified with its desirability as defined earlier – i.e. as the conditional expectation of utility given the performance of the action. We can therefore express his theory of choice thus:

Jeffrey Of all the prospects that constitute options for you, pick one with greatest desirability.

Although Jeffrey's theory has been very influential in philosophy, it has few contemporary supporters, with the majority of philosophical decision theorists accepting the objections to it lodged by causal decision theory.