Nagel would say that our conception of ourselves as temporally extended beings justifies prudence: acting in furtherance of reasons we think will belong to our future selves. Many prudential acts will limit the options our future selves have to those our past selves could predict wanting. Perhaps this slows our rate of change.

We must regard ourselves as somewhat temporally extended in order to get anyhting done at all – even cooking dinner. But at some point, we may find we have little in common with some of “us” who have gone before. In a sloppy effort to reconcile these two impressions, we may from time to time evolve into different “people” who nonetheless cooperate to produce a single narrative: a life.

Viewed this way (whimsically, as a sort of group project), it may make sense to say that M2 has a duty to reciprocate M1′s cooperative effort by finishing off, integrating, or otherwise honoring past projects. After all, I couldn’t have a life without her.

And, of course, we do tend to do this. We visit graves and old high schools, tell childhood stories and keep souveniers. Sometimes, we do something only because we know that we once would’ve lept at the chance.

Notice that economics acknowledges no such value! I looked, wide-eyed, at the guy who said this and responded, “I won’t tell anyone in the econ department. They would encourage you to ignore sunk costs!”

Suppose we’re obligated by past agreements, even those some distance in the past. That is, if I made a contract two years ago, you did your part, and now it’s come due for me to perform, I’m bound by that contract. Suppose over those two years you’ve actually changed a fair amount, such that your interests are different–maybe even such that you’d not make such an agreementnow, possibly even regret it. Call you two years ago M1, and you now M2. Suppose again we think the other party, P, has a right to your performance of the agreement. He has it by virtue of M1′s consent; absent that you’d have no duty to P. But that can only be the case if M1 is entitled to obligate M2; you couldn’t be under a duty just because any old person had agreed on your behalf, and you’re not obligated to do just ANYTHING, but only what M1 agreed to. So it can only be by virtue of a duty M2 owes M1 that the duty that M2 owes anything to P.

That’s pretty off-the-cuff, so I’m not sure it follows. Though if it DOES follow, maybe it’s just a reductio of the premise that we’re bound by past agrements.