A---It outweighs – bigger, irreversible, and intensifies genocide.

Third, any notion of risk must involve some kind of probability. What kind is involved in existential risk? Understanding the probability in terms of objective long-run frequencies won’t work, as the existential catastrophes we are concerned with can only ever happen once, and will always be unprecedented until the moment it is too late. We can’t say the probability of an existential catastrophe is precisely zero just because it hasn’t happened yet.

Situations like these require an evidential sense of probability, which describes the appropriate degree of belief we should have on the basis of the available information. This is the familiar type of probability used in courtrooms, banks and betting shops. When I speak of the probability of an existential catastrophe, I will mean the credence humanity should have that it will occur, in light of our best evidence.9

There are many utterly terrible outcomes that do not count as existential catastrophes.

One way this could happen is if there were no single precipitous event, but a multitude of smaller failures. This is because I take on the usual sense of catastrophe as a single, decisive event, rather than any combination of events that is bad in sum. If we were to squander our future simply by continually treating each other badly, or by never getting around to doing anything great, this could be just as bad an outcome but wouldn’t have come about via a catastrophe.

Alternatively, there might be a single catastrophe, but one that leaves open some way for humanity to eventually recover. From our own vantage, looking out to the next few generations, this may appear equally bleak. But a thousand years hence it may be considered just one of several dark episodes in the human story. A true existential catastrophe must by its very nature be the decisive moment of human history—the point where we failed.

Even catastrophes large enough to bring about the global collapse of civilization may fall short of being existential catastrophes. While colloquially referred to as “the end of the world,” a global collapse of civilization need not be the end of the human story. It has the required severity, but may not be permanent or irrevocable.

In this book, I shall use the term civilization collapse quite literally, to refer to an outcome where humanity across the globe loses civilization (at least temporarily), being reduced to a pre-agricultural way of life. The term is often used loosely to refer merely to a massive breakdown of order, the loss of modern technology, or an end to our culture. But I am talking about a world without writing, cities, law, or any of the other trappings of civilization.

This would be a very severe disaster and extremely hard to trigger. For all the historical pressures on civilizations, never once has this happened—not even on the scale of a continent.10 The fact that Europe survived losing 25 to 50 percent of its population in the Black Death, while keeping civilization firmly intact, suggests that triggering the collapse of civilization would require more than 50 percent fatality in every region of the world.11

Even if civilization did collapse, it is likely that it could be re-established. As we have seen, civilization has already been independently established at least seven times by isolated peoples.12 While one might think resource depletion could make this harder, it is more likely that it has become substantially easier. Most disasters short of human extinction would leave our domesticated animals and plants, as well as copious material resources in the ruins of our cities—it is much easier to re-forge iron from old railings than to smelt it from ore. Even expendable resources such as coal would be much easier to access, via abandoned reserves and mines, than they ever were in the eighteenth century.13 Moreover, evidence that civilization is possible, and the tools and knowledge to help rebuild, would be scattered across the world.

There are, however, two close connections between the collapse of civilization and existential risk. First, a collapse would count as an existential catastrophe if it were unrecoverable. For example, it is conceivable that some form of extreme climate change or engineered plague might make the planet so inhospitable that humanity would be irrevocably reduced to scattered foragers.14 And second, a global collapse of civilization could increase the chance of extinction, by leaving us more vulnerable to subsequent catastrophe.

One way a collapse could lead to extinction is if the population of the largest remaining group fell below the minimum viable population—the level needed for a population to survive. There is no precise figure for this, as it is usually defined probabilistically and depends on many details of the situation: where the population is, what technology they have access to, the sort of catastrophe they have suffered. Estimates range from hundreds of people up to tens of thousands.15 If a catastrophe directly reduces human population to below these levels, it will be more useful to classify it as a direct extinction event, rather than an unrecoverable collapse. And I expect that this will be one of the more common pathways to extinction.

We rarely think seriously about risks to humanity’s entire potential. We encounter them mostly in action films, where our emotional reactions are dulled by their overuse as an easy way to heighten the drama.16 Or we see them in online lists of “ten ways the world could end,” aimed primarily to thrill and entertain. Since the end of the Cold War, we rarely encounter sober discussions by our leading thinkers on what extinction would mean for us, our cultures or humanity.17 And so in casual contexts people are sometimes flippant about the prospect of human extinction.

But when a risk is made vivid and credible—when it is clear that billions of lives and all future generations are actually on the line—the importance of protecting humanity’s longterm potential is not, for most people, controversial. If we learned that a large asteroid was heading toward Earth, posing a greater than 10 percent chance of human extinction later this century, there would be little debate about whether to make serious efforts to build a deflection system, or to ignore the issue and run the risk. To the contrary, responding to the threat would immediately become one of the world’s top priorities. Thus our lack of concern about these threats is much more to do with not yet believing that there are such threats, than it is about seriously doubting the immensity of the stakes.

Yet it is important to spend a little while trying to understand more clearly the different sources of this importance. Such an understanding can buttress feeling and inspire action; it can bring to light new considerations; and it can aid in decisions about how to set our priorities.

LOOKING TO THE PRESENT

Not all existential catastrophes involve human extinction, and not all methods of extinction involve pain or untimely death. For example, it is theoretically possible that we could all simply decide not to reproduce. This could destroy our potential without, let us suppose, causing any suffering. But the existential risks we actually face are not so peaceful. Rather, they are obviously horrific by the most familiar moral standards.

If, over the coming century, humanity is destroyed in a nuclear winter, or an engineered pandemic, or a catastrophic war involving some new technology, then seven billion lives would be cut short—including, perhaps, your own life, or the lives of those you love. Many would likely die in agony—starving, or burning, or ravaged by disease.

The moral case for preventing such horror needs little elaboration. Humanity has seen catastrophes before, on smaller scales: thousands, or millions, of human lives destroyed. We know how tremendously important it is to prevent such disasters. At such a scale, we lose our ability to fully comprehend the magnitude of what is lost, but even then the numbers provide a guide to the moral stakes.18 Other things being equal, millions of deaths must be much worse than thousands of deaths; and billions, much worse than millions. Even measured just in terms of lives cut short, human extinction would easily be the worst event in our long history.

LOOKING TO OUR FUTURE

But an existential catastrophe is not just a catastrophe that destroys a particularly large number of lives. It destroys our potential.

My mentor, Derek Parfit, asked us to imagine a devastating nuclear war killing 99 percent of the world’s people.19 A war that would leave behind a dark age lasting centuries, before the survivors could eventually rebuild civilization to its former heights; humbled, scarred—but undefeated.

Now compare this with a war killing a full 100 percent of the world’s people. This second war would be worse, of course, but how much worse? Either war would be the worst catastrophe in history. Either would kill billions. The second war would involve tens of millions of additional deaths, and so would be worse for this reason. But there is another, far more significant difference between the two wars. Both wars kill billions of humans; but the second war kills humanity. Both wars destroy our present; but the second war destroys our future.

It is this qualitative difference in what is lost with that last percent that makes existential catastrophes unique, and that makes reducing the risk of existential catastrophe uniquely important.20

In expectation, almost all humans who will ever live have yet to be born. Absent catastrophe, most generations are future generations. As the writer Jonathan Schell put it:

The procession of generations that extends onwards from our present leads far, far beyond the line of our sight, and, compared with these stretches of human time, which exceed the whole history of the earth up to now, our brief civilized moment is almost infinitesimal. Yet we threaten, in the name of our transient aims and fallible convictions, to foreclose it all. If our species does destroy itself, it will be a death in the cradle—a case of infant mortality.21

And because, in expectation, almost all of humanity’s life lies in the future, almost everything of value lies in the future as well: almost all the flourishing; almost all the beauty; our greatest achievements; our most just societies; our most profound discoveries.22 We can continue our progress on prosperity, health, justice, freedom and moral thought. We can create a world of wellbeing and flourishing that challenges our capacity to imagine. And if we protect that world from catastrophe, it could last millions of centuries. This is our potential—what we could achieve if we pass the Precipice and continue striving for a better world.

It is this view of the future—the immense value of humanity’s potential—that most persuades me to focus my energies on reducing existential risk. When I think of the millions of future generations yet to come, the importance of protecting humanity’s future is clear to me. To risk destroying this future, for the sake of some advantage limited only to the present, seems to me profoundly parochial and dangerously short-sighted. Such neglect privileges a tiny sliver of our story over the grand sweep of the whole; it privileges a tiny minority of humans over the overwhelming majority yet to be born; it privileges this particular century over the millions, or maybe billions, yet to come.23

To see why this would be wrong, consider an analogy with distance. A person does not matter less, the further away from you they are in space. It matters just as much if my wife gets sick while she is away at a conference in Kenya as if she gets sick while home with me in Oxford. And the welfare of strangers in Kenya matters just as much as the welfare of strangers in Oxford. Of course, we may have special duties to some individuals—to family; to members of the same community—but it is never spatial distance, in itself, that determines these differences in our obligations. Recognizing that people matter equally, regardless of their geographic location, is a crucial form of moral progress, and one that we could do much more to integrate into our policies and our philanthropy.

People matter equally regardless of their temporal location too. Our lives matter just as much as those lived thousands of years ago, or those a thousand years hence.24 Just as it would be wrong to think that other people matter less the further they are from you in space, so it is to think they matter less the further away from you they are in time. The value of their happiness, and the horror of their suffering, is undiminished.

Recognizing that people matter equally, wherever they are in time, is a crucial next step in the ongoing story of humanity’s moral progress. Many of us recognize this equality to some extent already. We know it is wrong to make future generations worse off in order to secure lesser benefits for ourselves. And if asked, we would agree that people now don’t objectively matter more than people in the future. But we assume that this leaves most of our priorities unaltered. For example, thinking that long-run effects of our choices quickly disappear; that they are so uncertain that the good cancels the bad; or that people in the future will be much better situated to help themselves.25

But the possibility of preventable existential risks in our lifetimes shows that there are issues where our actions can have sustained positive effects over the whole longterm future, and where we are the only generation in a position to produce those effects.26 So the view that people in the future matter just as much as us has deep practical implications. We have a long way to go if we are to understand these and integrate them fully into our moral thinking.

Considerations like these suggest an ethic we might call longtermism, which is especially concerned with the impacts of our actions upon the longterm future.27 It takes seriously the fact that our own generation is but one page in a much longer story, and that our most important role may be how we shape—or fail to shape—that story. Working to safeguard humanity’s potential is one avenue for such a lasting impact and there may be others too.28

One doesn’t have to approach existential risk from this direction—there is already a strong moral case just from the immediate effects—but a longtermist ethic is nevertheless especially well suited to grappling with existential risk. For longtermism is animated by a moral re-orientation toward the vast future that existential risks threaten to foreclose.

Of course, there are complexities.

When economists evaluate future benefits, they use a method called discounting, which dampens (“discounts”) benefits based on how far away they are in time. If one took a commonly used discount rate of 5 percent per year and applied it to our future, there would be strikingly little value left. Applied naïvely, this discount rate would suggest our entire future is worth only about twenty times as much as the coming year, and that the period from 2100 to eternity is worth less than the coming year. Does this call into question the idea that our future is extremely valuable?

No. Results like this arise only from an incorrect application of the economic methods. When the subtleties of the problem are taken into account and discounting is correctly applied, the future is accorded an extremely high value. The mathematical details would take us too far afield, but for now it suffices to note that discounting human wellbeing (as opposed to instrumental goods such as money), purely on the basis of distance away from us in time, is deeply implausible—especially over the long time periods we are discussing. It implies, for example, that if you can save one person from a headache in a million years’ time, or a billion people from torture in two million years, you should save the one from a headache.29 A full explanation of why economic discounting does not trivialize the value of the longterm future can be found in Appendix A.

Some philosophers question the value of protecting our longterm future for quite a different reason. They note that the timing of the benefits is not the only unusual feature of this case. If we save humanity from extinction, that will change the number of people who will ever live. This brings up ethical issues that don’t arise when simply saving the lives of existing people. Some of the more extreme approaches to this relatively new field of “population ethics” imply that there is no reason to avoid extinction stemming from considerations of future generations—it just doesn’t matter whether these future people come into being or not.

A full treatment of these matters would take too long and be of interest only to a few, so I reserve the detailed discussion for Appendix B. To briefly summarize: I do not find these views very plausible, either. They struggle to capture our reasons to care about whether we make future lives worse by polluting the planet, or changing the climate, and to explain why we have strong reasons to prevent terrible lives from existing in the future. And all but the most implausible of these views agree with the immense importance of saving future generations from other kinds of existential catastrophe, such as the irrevocable collapse of civilization. Since most things that threaten extinction threaten such a collapse too, there is not much practical difference. That said, the issues are complex, and I encourage interested readers to consult the appendix for details.

There is one other objection I want to touch on. When I was younger, I sometimes took comfort in the idea that perhaps the outright destruction of humanity would not be bad at all. There would be no people to suffer or grieve. There would be no badness at those future times, so how could the destruction be bad? And if the existence of humanity was somehow essential to judgments of right and wrong, good and bad, then perhaps such concepts would fail to apply at all in the stillness that followed.

But I now see that this is no better than the old argument by the philosopher Epicurus that your death cannot be bad for you, since you are not there to experience it. What this neglects is that if I step out into the traffic and die, my life as a whole will be shorter and thereby worse: not by having more that is bad, but by containing less of everything that makes life good. That is why I shouldn’t do it. While Epicurus’s argument may provide consolation in times of grief or fear, it is not fit to be a guide for action, and no one treats it so. Imagine a government using it as the basis for our policies on safety or healthcare—or for our laws on murder.

If a catastrophe this century were to cause our extinction, then humanity’s life would be shorter and thereby worse.30 Given that we may just be in our infancy, it would be much shorter; much worse. Even if there were no one remaining to judge this as a tragedy, we can rightly judge it so from here. Just as we can judge events in other places, so we can judge events in other times.31 And if these judgments are correct now, they shall remain correct when we are no more. I wouldn’t blame people who, in humanity’s final hours, found consolation in such Epicurean arguments. But the length and quality of humanity’s life is still ours to decide, and we must own this responsibility.32

These are not the only possible objections. Yet we need not resolve every philosophical issue about the value of the future in order to decide whether humanity’s potential is worth protecting. For the idea that it would be a matter of relative indifference whether humanity goes extinct, or whether we flourish for billions of years, is, on its face, profoundly implausible. In this sense, any theory that denies it should be subject to significant skepticism.33

What’s more, the future is not the only moral lens through which to view existential catastrophe. It is the one that grips me most, and that most persuades me to devote my time and energy to this issue, but there are other lenses, drawing on other moral traditions. So let us briefly explore how concern about existential risk could also spring from considerations of our past, our character and our cosmic significance. And thus how people with many different understandings of morality could all end up at this common conclusion.

We are not the first generation. Our cultures, institutions and norms; our knowledge, technology and prosperity; these were gradually built up by our ancestors, over the course of ten thousand generations. In the last chapter we saw how humanity’s remarkable success has relied on our capacity for intergenerational cooperation: inheriting from our parents, making some small improvements of our own, and passing it all down to our children. Without this cooperation we would have no houses or farms, we would have no traditions of dance or song, no writing, no nations.34

This idea was beautifully expressed by the conservative political theorist Edmund Burke. In 1790 he wrote of society:

It is a partnership in all science; a partnership in all art; a partnership in every virtue, and in all perfection. As the ends of such a partnership cannot be obtained except in many generations, it becomes a partnership not only between those who are living, but between those who are living, those who are dead, and those who are to be born.35

This might give us reasons to safeguard humanity that are grounded in our past—obligations to our grandparents, as well as our grandchildren.

Our ancestors set in motion great projects for humanity that are too big for any single generation to achieve. Projects such as bringing an end to war, forging a just world and understanding our universe. In the year 65 CE, Seneca the Younger explicitly set out such a vast intergenerational project:

The time will come when diligent research over long periods will bring to light things which now lie hidden. A single lifetime, even though entirely devoted to the sky, would not be enough for the investigation of so vast a subject… And so this knowledge will be unfolded only through long successive ages. There will come a time when our descendants will be amazed that we did not know things that are so plain to them… Let us be satisfied with what we have found out, and let our descendants also contribute something to the truth… Many discoveries are reserved for ages still to come, when memory of us will have been effaced.36

It is astounding to be spoken to so directly across such a gulf of time, and to see this 2,000-year plan continue to unfold.37

A human, or an entire generation, cannot complete such grand projects. But humanity can. We work together, each generation making a little progress while building up the capacities, resources and institutions to empower future generations to take the next step.

Indeed, when I think of the unbroken chain of generations leading to our time and of everything they have built for us, I am humbled. I am overwhelmed with gratitude; shocked by the enormity of the inheritance and at the impossibility of returning even the smallest fraction of the favor. Because a hundred billion of the people to whom I owe everything are gone forever, and because what they created is so much larger than my life, than my entire generation.

The same is true at the personal level. In the months after my daughter was born, the magnitude of everything my parents did for me was fully revealed. I was shocked. I told them; thanked them; apologized for the impossibility of ever repaying them. And they smiled, telling me that this wasn’t how it worked—that one doesn’t repay one’s parents. One passes it on.

My parents aren’t philosophers. But their remarks suggest another way in which the past could ground our duties to the future. Because the arrow of time makes it so much easier to help people who come after you than people who come before, the best way of understanding the partnership of the generations may be asymmetrical, with duties all flowing forwards in time—paying it forwards. On this view, our duties to future generations may thus be grounded in the work our ancestors did for us when we were future generations.38

So if we drop the baton, succumbing to an existential catastrophe, we would fail our ancestors in a multitude of ways. We would fail to achieve the dreams they hoped for; we would betray the trust they placed in us, their heirs; and we would fail in any duty we had to pay forward the work they did for us. To neglect existential risk might thus be to wrong not only the people of the future, but the people of the past.

It would also be to risk the destruction of everything of value from the past we might have reason to preserve.39 Some philosophers have suggested that the right way to respond to some valuable things is not to promote them, but to protect or preserve them; to cherish or revere them.40 We often treat the value of cultural traditions in this way. We see indigenous languages and ways of life under threat—perhaps to be lost forever to this world—and we are filled with a desire to preserve them, and protect them from future threats.

Someone who saw the value of humanity in this light may not be so moved by the loss of what could have been. But they would still be horrified by extinction: the ruin of every cathedral and temple, the erasure of every poem in every tongue, the final and permanent destruction of every cultural tradition the Earth has known. In the face of serious threats of extinction, or of a permanent collapse of civilization, a tradition rooted in preserving or cherishing the richness of humanity would also cry out for action.41

Finally, we might have duties to the future arising from the flaws of the past. For we might be able to make up for some of our past wrongs. If we failed now, we could never fulfill any duties we might have to repair the damage we have done to the Earth’s environment—cleaning up our pollution and waste; restoring the climate to its pre-industrial state; returning ecosystems to their vanished glory. Or consider that some of the greatest injustices have been inflicted not by individuals upon individuals, but by groups upon groups: systematic persecution, stolen lands, genocides. We may have duties to properly acknowledge and memorialize these wrongs; to confront the acts of our past. And there may yet be ways for the beneficiaries of these acts to partly remedy them or atone for them. Suffering an existential catastrophe would remove any last chance to do so.