Desire-as-Belief Implies Opinionation or Indifference

Rationalizations of deliberation often make reference to two kinds of mental state, which we call belief and desire. It is worth asking whether these kinds are necessarily distinct, or whether it might be possible to construe desire as belief of a certain sort — belief, say, about what would be good. An expected value theory formalizes our notions of belief and desire, treating each as a matter of degree. In this context the thesis that desire is belief might amount to the claim that the degree to which an agent desires any proposition A equals the degree to which the agent believes the proposition that A would be good. We shall write this latter proposition ‘A◦’ (pronounced ‘A halo’). The Desire-as-Belief Thesis states, then, that to each proposition A there corresponds another proposition A◦, where the probability of A◦ equals the expected value of A.