 The desire papers (1988a, 1996b) are also about the Humean view that motivation requires both a belief and a desire. Lewis aims to attack the anti-Humean position that some beliefs, in particular beliefs that a certain thing is good, can play the functional roles of both beliefs and desires. He argues that this is not, in general, possible. And the argument is that beliefs and desires update in different ways. Or, at least, that anyone who updates their beliefs by conditionalisation and updates their valuation functions in a plausible way, will not be able to preserve a perfect correlation between their desire for a proposition being true and their belief in that proposition’s goodness.