Many people claim that fulfilling a person’s desires after they are dead cannot benefit that person. Not just that it isn’t strategic to honour such desires, but that such desires can’t be fulfilled. Either it is not really possible for a person to desire for things that won’t happen until after they are dead, or it is somehow not possible for such a desire to be fulfilled.
It seems possible (i.e. consistent) to have a concept of preference fulfilment that requires the person to be alive at the time the preference is fulfilled, and similarly possible to have one that doesn’t require this. Which is the ‘true’ concept seems a poorly defined question. Which concept you choose matters though. The concept is intended to fit into your ethical stance, or your preferences, so presumably one concept better fits what should really happen, or what you really want. Here are some considerations for working out which concept you want to use:
The analogy with space
If you can’t have your preferences fulfilled in parts of time where you don’t exist (after you die, or before you are born), can you have preferences fulfilled in parts of space where you don’t exist? Can I prefer for my fridge to contain milk even before I open the door? Can I prefer my brother to be alive even though we don’t cohabit the same spatial region? If so, what’s the relevant difference between time and space? You can bite the bullet and refuse to acknowledge preferences over anything other than a person’s own mental states. And you could do that without straying too far from your intuitions about what you do prefer by noting that a lot of things that are spatially distant from you will eventually send some kind of signal to you, and claim it is those signals you care about. You are still then committed to indifference about for instance what kinds of assault go on behind the closed doors of people you love, as long as you are never informed about them.
The vagueness of personal identity
Another implication of not being able to value things that don’t overlap with you in time is that what you can or can’t value depends on what counts as ‘you’. And what counts as ‘you’ is pretty vaguely defined usually. Some people think ‘you’ are the bunch of physical processes we call ‘you’, or whichever of those we find most important – your continued memory and personality for instance. This is a concept with pretty blurry boundaries, and different people find different features important enough to call ‘themselves’. Do you want such an arbitrary definitional choice to determine what values can count for anything? If you think of me as a series of person-moments, suddenly I can’t legitimately care about the milk in the fridge even if a later-Katja will learn about it later. If you identify me with all past and future people who feel a lot like me, then I’m allowed preferences about what happens after the death of this body. Is there some particular line in the many-dimensional space of things more or less like me that seems hugely important to you in deciding which preferences are valid?
Other people think there is more to ‘you’ than a set of physical processes, in which case there may be one clear line around what counts as ‘you’. On the other hand, you probably don’t have any good way to locate this non-physical line. The more ignorant you are about the location of the line, the more you are committed to caring about preferences that may be outside it, assuming some straightforward kind of consequentialism.
The limits of biology
Perhaps you want a concept of preference fulfilment that requires the person to be alive at the time because you doubt anyone can actually have preferences that don’t involve their existing. For instance perhaps you think the way that preferences are encoded in a mind involve a representation of oneself enjoying the thing, or something else like that. So that even if I say ‘I value bandicoots existing’, my feelings are really that I value thinking that bandicoots exist. I’ll discuss that view in more depth if anyone who actually holds this view tells me exactly what it is. At a glance it seems this kind of view also implies that it’s impossible to care about anything other than your mental states.
For the purpose of trading, the more of another person’s preferences you are willing to deal with, the better for you. But this is a different question to which of their values you want to care about outside of trading.