If reason alone cannot be a motive to any action of the will, then an agent who is locked into a preference order of 01, 11, 00, 10 is not helped by reflecting that he might get on better by acting as if he had a different order. Yet the theory of rational choice is not committed to the common presumption that only current desires can move a person. In this chapter I shall argue the case for construing the proposition that action results from desire plus belief in such a way that belief can be a motive to the will. Rational agents act from objectively good reasons, whose merits are conceptually independent of their current desires. When ‘objectively good reasons’ is taken as I shall propose, it becomes possible for a rational prisoner to escape from the Leviathan trap.

The key question is whether the rationality of action is always relative to the current desires of the agent. It is an old one, whose recent battle lines derive from Hume and Kant. What form must a reason for action take if it is to move a man? The Humean camp holds that reasons for action must be ‘internal’ to the desires of the agent, whose reasons they are. The Kantian holds that ‘external’ reasons can be effective. This classic dispute involves whole rival philosophies of mind but turns on a very precise point. Indeed the point is so precise and well-worked that the issue resembles the disputed analysis of a chess opening, with each camp having a favoured line and a counter to the other's.