What principles should govern human action? As rational beings, we should act rationally. As moral beings, we should act morally. What, in each case, are the principles involved? What is it to act rationally, or morally? It is often thought, or said, that philosophers are preeminently the people who have (and have neglected) a moral obligation to apply their rational skills to these great questions. Derek Parfit, fellow of All Souls at Oxford, is a philosopher who does exactly this. He marshals the most immediately obvious and plausible principles of rational, and of moral, action; he submits them, with great dialectical skill, to patient and thorough analysis and criticism; he tests the results against our intuitions, and draws the consequences.
He does more. One of the metaphysical questions that have engaged the attention of philosophers, at least from Descartes onward, is that of the nature of personal identity. Most of us are greatly concerned with our survival in this life as just the person that each of us is. Parfit argues that this concern with identity is misplaced; that what really does, or should, matter to us is something else, something that is indeed, in some degree, an element in such survival, but is, nevertheless, to be distinguished from the preservation of personal identity. And he argues further that once this truth is appreciated, and our ordinary beliefs revised accordingly, not only will our understanding of the principles of rational and moral action be modified, but the result will be, in certain additional ways, better for us. At least, he claims, it is better for him. But he admits that there are great obstacles in the way of our appreciating this truth.
Parfit delays consideration of this thesis until he has examined and criticized certain immediately obvious candidates for the status of acceptable general principles of rational, or of moral, action. One venerable theory about rationality is the “self-interest theory” which has been elaborated by philosophers since Hobbes. This is the view that each person should, if he is to act rationally, always act in such a way as to secure the best results for himself throughout his entire life; to secure, that is, that his life goes, as a whole, as well as possible. A rival to this view is the “present-aim theory”; according to this, it is rational for each person always to act in the way that will best achieve his current aims, as distinguished from the goals he may have in later life. Evidently either theory allows for great variation in content, depending on different conceptions of self-interest. I may believe, for example, that my self-interest is in a life of pleasure or that it is in a life in which my abilities are fully developed; my “present aims” may similarly differ.
Trivially and uninterestingly, the two theories might even coincide: e.g., if an agent’s sole present aim were to achieve the goal of the self-interest theorist. But generally, and more interestingly, they will …