Thomas Nagel tries to answer this question: how is it possible that "there is reason to do not only what will promote that for which there is presently a reason, but also what will promote something future for which it is expected there will be a reason" (p. 36)? Nagel describes the method he uses to answer this question as a "method ... of metaphysical ethics: moral and other practical requirements are grounded ... in a metaphysics of the person" (p. 18). The particular account of prudence Nagel gives using this method is, roughly, that prudence is that practical expression of a person's conception of himself as equally real over time.
I am first going to lay out what I take to be the important steps of Nagel's argument for this account of prudence, and then I will try to show that this account produces a type of counterintuitive result. After showing what it is in Nagel's account that produces this result, I will make what I think is the best possible revision of the account. Finally, I will argue that there may be a fatal flaw even in the revision, and suggest that if the flaw is fatal, then the "method of metaphysical ethics" cannot explain the possibility of prudence.
If someone is to conceive of himself as a person, Nagel begins, part of his conception should acknowledge his persistence as a person over time. If someone conceived of himself as a person but could acknowledge his personhood only at, say, the present moment, then his conception would be so degenerate that it would merit our attention only as a curiosity. If someone is to have a full-blown conception of himself as a person, he must be able to conceive of himself as "equally real over time" (p. 71). He must be able "to identify