Analysis: Desire as Belief

Desire as Belief

Lewis claims to reduce to absurdity the supposition that (some) desires are belief-like. He construes and attempts to refute the claim within a Bayesian framework (Jeffrey 19983), assuming that agents assign probabilistic degrees of belief (credences) and values to propositions, which jointly satisfy the following axiom ADDITIVITY, where the Ai’s are a partition of A:

V(A)= Ei V(Ai).P(A) / Ei P(Ai)

The value of a state of affairs, A, is the probability-weighted average of the values of all the ways it can come about.

Lewis assumes, harmlessly simplifying, that there are just two objective values (1 and 0), depending on whether a state of affairs is good or bad. The supposition that (some)desires are belief-like is now rendered as the claim that there is a function that assigns to each proposition.

Bayesian decision theory in conjunction with Desire-as-Belief, imposes two different constraints on the value a rational agent may ascribe to the truth of a given proposition. The two constraints are incompatible. Lewis claims the culprit is Desire-as-Belief. ‘Decision theory is an intuitively convincing and well worked-out formal theory of belief, desire, and what it means to serve our desires according to our beliefs……if an anti-Humean Desire-as-Belief Thesis collides with Decision Theory, it is the Desire-as-Belief Thesis that must go’ (Lewis 1988:325).

Lewis suggests that the desire-as-belief doctrine has met ethical implications. If there were some propositions belief or disbelief in which was necessarily connected with desire, some of them presumably would be true; then we surely would want to say that the true ones were the objective truth about ethical reality (1996:60).

…..it might seem as if desire-as-belief isn’t necessary for objectivism. Even if there are evaluative propositions, they might not align properly with an agent’s motivations, since agents can act against their evaluative judgement. Almost everyone admits the occurrence of weakness of will. Some even go further. Evaluative beliefs, they claim, can be motivationally inert. Whereas the akratic agent is insufficiently motivated by his judgement, the immoralist is perfectly indifferent to moral considerations.

Desire as Belief, Lewis notwithstanding, ANALYSIS, Vol 67, No 294, April 2007, Blackwell publishing