Part two: Rationality and time

Abstract

The second part of Reasons and Persons, in which Parfit argues against the Self-Interest Theory of rationality. Self-interest theorists hold that differences between persons are supremely important but that differences between the same person at different times are not. Parfit argues that this asymmetry makes self-interest theory vulnerable to attack from two directions: from the side of morality and from the side of present-aim theory. He shows that arguments a self-interest theorist uses to explain why it is irrational to act against one’s long-term self-interest can be turned against self-interest theory and used as arguments in favor of morality, while arguments against morality can be used to support present-aim theory.