In an early article, Gauthier argued that, to act rationally, we must act morally. I tried to refute that argument. Since Gauthier was not convinced, I shall try again.
Gauthier assumes that, to be rational, we must maximize our own expected utility. Though he distinguishes between ‘utility’ and ‘benefit’, this distinction does not affect his main arguments. We can regard him as appealing to the Self-interest Theory.
Many writers have argued that, in self-interested terms, it is always rational to act morally. According to most of these writers, morality and self-interest coincide. But that is not Gauthier's line. Gauthier concedes that acting morally may be, and be known to be, worse for us. He claims that, even in such cases, it is rational to act morally.
If we appeal to the Self-interest Theory, it may seem impossible to defend that claim. How can our acts be rational, in self-interested terms, if we know them to be worse for us? But Gauthier revises the Self-interest Theory. On the standard version of this theory, an act is rational if it will maximize our expected benefit – or be expectably-best for us. On Gauthier's version, we should aim to benefit ourselves not with our acts but only with our dispositions. A disposition is rational if having it will be expectably-best for us. An act is rational if it results from such a disposition.
Besides revising the Self-interest Theory, Gauthier restricts the scope of morality. To act morally, Gauthier claims, we must honour our agreements.