I believe in life after death.

No, I don’t think that I will live on as a conscious being after my earthly demise. I’m firmly convinced that death marks the unqualified and irreversible end of our lives.

My belief in life after death is more mundane. What I believe is that other people will continue to live after I myself have died. You probably make the same assumption in your own case. Although we know that humanity won’t exist forever, most of us take it for granted that the human race will survive, at least for a while, after we ourselves are gone.

Because we take this belief for granted, we don’t think much about its significance. Yet I think that this belief plays an extremely important role in our lives, quietly but critically shaping our values, commitments and sense of what is worth doing. Astonishing though it may seem, there are ways in which the continuing existence of other people after our deaths — even that of complete strangers — matters more to us than does our own survival and that of our loved ones.

Consider a hypothetical scenario. Suppose you knew that although you yourself would live a long life and die peacefully in your sleep, the earth and all its inhabitants would be destroyed 30 days after your death in a collision with a giant asteroid. How would this knowledge affect you?

If you are like me, and like most people with whom I have discussed the question, you would find this doomsday knowledge profoundly disturbing. And it might greatly affect your decisions about how to live. If you were a cancer researcher, you might be less motivated to continue your work. (It would be unlikely, after all, that a cure would be found in your lifetime, and even it were, how much good would it do in the time remaining?) Likewise if you were an engineer working to improve the seismic safety of bridges, or an activist trying to reform our political or social institutions or a carpenter who cared about building things to last. What difference would these endeavors make, if the destruction of the human race was imminent?

If you were a novelist or playwright or composer, you might see little point in continuing to write or compose, since these creative activities are often undertaken with an imagined future audience or legacy in mind. And faced with the knowledge that humanity would cease to exist soon after your death, would you still be motivated to have children? Maybe not.

Notice that people do not typically react with such a loss of purpose to the prospect of their own deaths. Of course, many people are terrified of dying. But even people who fear death (and even those who do not believe in a personal afterlife) remain confident of the value of their activities despite knowing that they will die someday. Thus there is a way in which the survival of other people after our deaths matters more to us than our own survival.

The explanation for this may seem simple: if the earth will be destroyed 30 days after we die, then everyone we care about who is alive at that time will meet a sudden, violent end. Spouses and partners, children and grandchildren, friends and lovers: all would be doomed. Perhaps it is our concern for our loved ones that explains our horror at the prospect of a post-mortem catastrophe.

But I don’t think this is the full story. Consider another hypothetical scenario, drawn from P. D. James’s novel “The Children of Men.” In Ms. James’s novel, humanity has become infertile, with no recorded birth having occurred in over 25 years. Imagine that you found yourself living in such circumstances. Nobody now alive is younger than 25, and the disappearance of the human race is imminent as an aging population inexorably fades away. How would you react?

As in the case of the asteroidal collision, many activities would begin to seem pointless under these conditions: cancer research, seismic safety efforts, social and political activism and so on. Beyond that, as Ms. James’s novel vividly suggests, the onset of irreversible global infertility would be likely to produce widespread depression, anxiety and despair.

Some people would seek consolation in religious faith, and some would find it. Others would take what pleasure they could in activities that seemed intrinsically rewarding: listening to music, exploring the natural world, spending time with family and friends and enjoying the pleasures of food and drink. But even these activities might seem less fulfilling, and be tinged with sadness and pain, when set against the background of a dying humanity.

NOTICE that in this scenario, unlike that of the asteroidal collision, nobody would die prematurely. So what is dismaying about the prospect of living in an infertile world cannot be that we are horrified by the demise of our loved ones. (They would die eventually, of course, but that is no different from our actual situation.) What is dismaying is simply that no new people would come into existence.

This should give us pause. The knowledge that we and everyone we know and love will someday die does not cause most of us to lose confidence in the value of our daily activities. But the knowledge that no new people would come into existence would make many of those things seem pointless.

I think this shows that some widespread assumptions about human egoism are oversimplified at best. However self-interested or narcissistic we may be, our capacity to find purpose and value in our lives depends on what we expect to happen to others after our deaths. Even the egotistic tycoon who is devoted to his own glory might discover that his ambitions seemed pointless if humanity’s disappearance was imminent. Although some people can afford not to depend on the kindness of strangers, virtually everyone depends on the future existence of strangers.

There is also a lesson here for those who think that unless there is a personal afterlife, their lives lack any meaning or purpose. What is necessary to underwrite the perceived significance of what we do, it seems, is not a belief in the afterlife but rather a belief that humanity will survive, at least for a good long time.

But will humanity survive for a good long time? Although we normally assume that others will live on after we ourselves have died, we also know that there are serious threats to humanity’s survival. Not all of these threats are human-made, but some of the most pressing certainly are, like those posed by climate change and nuclear proliferation. People who worry about these problems often urge us to remember our obligations to future generations, whose fate depends so heavily on what we do today. We are obligated, they stress, not to make the earth uninhabitable or to degrade the environment in which our descendants will live.

I agree. But there is also another side to the story. Yes, our descendants depend on us to make possible their existence and well-being. But we also depend on them and their existence if we are to lead flourishing lives ourselves. And so our reasons to overcome the threats to humanity’s survival do not derive solely from our obligations to our descendants. We have another reason to try to ensure a flourishing future for those who come after us: it is simply that, to an extent that we rarely recognize or acknowledge, they already matter so much to us.

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Samuel Scheffler, a professor of philosophy and law at New York University, is the author of the forthcoming book “Death and the Afterlife.”