People exist at different times. My life, for instance, consists of me-at-age-five, me-as-a-teenager, me-as-a-university-student, and of course many other temporal stages (or time-slices) as well. In a sense, then, we can see a single person, whose life extends over time, as akin to a group of people, each of whom exists for just a short stretch of time.

This perspective raises a host of interesting questions. Here’s one: Coherence is often considered a rational virtue. Rational people have beliefs, desires, and other attitudes that cohere with each, that fit together nicely, while having attitudes that clash with each other is a mark of irrationality. Certainly that’s the case for coherence at a single time. If there’s a time where you have inconsistent beliefs, then that’s irrational. But what about coherence over time? Is that a rational virtue too? Should your beliefs and desires held at different times fit together in any particular way? Should your actions, performed at different times, fit together as part of a sensible long-term course of action?

Many philosophers have thought so. After all, if your beliefs, desires, and actions at different times don’t cohere with each other, then you’re likely to engage in self-defeating behavior. If you keep changing your mind about what to try to achieve or about how best to achieve it, then (so the thought goes) you’ll wind up achieving nothing. You will start projects but promptly abandon them as soon as you adopt different goals or change your views about the best way of reaching your goals.

But there are grounds for suspicion. To take the case of belief, why should what you believed in the past matter for what you ought to do now? Shouldn’t it just depend on what evidence you have right now? And letting facts about how you acted in the past influence your present decision-making can seem like committing the sunk-cost fallacy.

More to the point, why should you treat your past selves differently from how you treat other people? Why should your past selves’ attitudes and actions matter more for how you rationally ought to be going forward, than any stranger’s attitudes and actions? Seeing yourself as akin to a group (a group of time-slices), it’s tempting to think that your different time-slices are no more beholden to each others’ attitudes and actions than are the members of any other group.

Insights about individual rationality can help us design better group decision-making procedures, and insights about groups can lead us to revise our views about individual rationality.

Beyond demoting the importance of coherence over time, adopting this perspective means taking certain analogies seriously. We can see memory as a kind of testimony – testimony from one’s past selves. And whether one should trust one’s memory will depend on exactly how reliable one thinks it is, just as whether one should trust someone’s testimony depends on how reliable one thinks that person is. And we can see intentions as like commands or advice coming from one’s past selves. Whether one should satisfy that intention then depends on how good one thinks one’s past self was as an advisor, or on whether there are any costs to disobeying its commands. (A useful way to control one’s future selves is to change their incentives; think of the smoker who tries to quit in part by betting his friend that he won’t have a cigarette in the next month.)

This idea, that individuals are akin to groups, goes back at least to Derek Parfit. In his highly influential Reasons and Persons (OUP, 1986), he writes that “when we are considering both theoretical and practical rationality, the relation between a person now and himself at other times is relevantly similar to the relation between different people” (p. 190). Parfit thought that personal identity over time was unimportant both for rationality and, more controversially, for morality. (The latter raises some other interesting issues: If your past and future selves are like other people, as far as morality is concerned, is it morally wrong to coerce your future self, like our foresighted smoker, or is it sometimes morally permissible to coerce others, if it’s for a good reason? Is it wrong to harm one person to benefit her in the future, say by sticking her with a needle to vaccinate her, or is it sometimes morally permissible to harm one person to benefit a different person?)

Taking seriously the analogy between individuals and groups opens promising avenues for new research. There is a vibrant debate about group rationality, about how we should take individuals’ beliefs or preferences and aggregate them to come up with a ‘group belief’ or ‘group preference.’ This is important for democracy and for other cases where people must act as a unified group–that is, as akin to a single individual–as in the case of a panel of judges who must issue a joint opinion. Group Agency by Christian List and Philip Pettit, is an excellent discussion of these issues.

We might even go further and think of individuals at particular times–your different time-slices, say–as group-like in some respects. When you seem to have contradictory beliefs, perhaps it’s useful to think of yourself as a fragmented person, with each fragment having a different (but consistent!) set of beliefs. And in cognitive science, it is now common to think of the mind as consisting of different semi-autonomous subsystems (though whether these subsystems are like individuals varies depending on the case) – there is Marvin Minsky’s Society of Mind hypothesis, Jerry Fodor’s modularity hypothesis, and Kahneman and Tversky’s division of the mind into System 1 (which is fast, automatic, and subconscious) and System 2 (which is slow, effortful, and conscious).

Seeing individuals as groups, then, opens opportunities for cross-pollination. Insights about individual rationality can help us design better group decision-making procedures, and insights about groups can lead us to revise our views about individual rationality, for instance, by leading us to conclude that coherence over time isn’t a rational imperative. And, as Parfit himself thought, it can lead us to a more open, generous stance, where we treat the boundary between ourselves and others as less important than we otherwise might.

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