The Long View: The End of The World Book Review
John wrote a lot of book reviews. I based my own book reviews on what he did on his own site. This is one of his earliest, written in 1998. The influence was really all one way [from John to me], but you can see that we had lots of interests in common. For example, this bit of his review:
The Carter-Leslie Doomsday Argument makes most sense if you make some factual assumptions. First, you should assume that the number of people who will have ever lived will be finite. (This is reasonable but not inevitable: there are cosmological theories which make an infinity of future human beings a possibility, or even a necessity.) Second, you should assume that human population tends to increase geometrically over time, so that a historical graph of world population produces a Malthusian slope. Third, you should assume that the world is totally or substantially deterministic, so that events in the past have reasonably reliable implications for what happens in the future.
John was interested in science, but being a philosophically inclined lawyer, I mostly observed him to reason with words and concepts. One of the things I appreciate most about my education is that I learned to think visually, mathematically, and verbally. This was a happy accident, since I was the physics nerd who liked speech and debate.
Thus, when I see that Leslie assumed that human population increases geometrically, I think: no it doesn't, most animal populations, humans included, increase logistically. And then I plot it. Exponential curves look a hell of a lot like exponentials at the beginning, especially when you take into account random noise, but in the long run they look a lot different.
Logistic Curve vs Exponential Curve
On the other hand, I think John was a better prose stylist than me, and his writing was more engaging. So maybe he was on to something.
The End of the World:
The Science and Ethics of Human Extinction
by John Leslie
Routledge, 1996
310 pages, $28:00 (Hardcover)
$16.99 (Paperback)
ISBN: 0-415-14043-9
This book [Amazon link] explains the Carter-Leslie Doomsday Argument, which purports to offer sound mathematical reasons for supposing that the human race will become extinct in a century or two. The Argument evolved dialectically:
[Thesis] In the beginning of modern science was the Copernican Principle, which counsels that observers should be skeptical of claims that they are observing from a privileged position. Thus, though the sun and stars may appear to revolve around the Earth, think twice before you decide you are at the center of the universe.
[Antithesis] By the last quarter of the twentieth century, many scientists had nevertheless concluded that we were in fact living in a privileged world. Our universe is governed by a small set of physical constants, whose values appear to be arbitrary. Almost all values for those numbers would produce universes of nothing but black holes or radiation. Only one set of numbers (within very narrow limits) produces stars and chemistry and biology, and that is the set we have. Similarly, there was thought to be some reason for supposing that the appearance of intelligent life on Earth was the outcome of a series of vanishingly improbable accidents. Yet, here we are, worrying about it. Brandon Carter, the Cambridge mathematician, coined the term "the Anthropic Principle" to describe the qualification of the Copernican Principle that the unlikely nature of our world seemed to require. The Principle states that an observer (such as the human race collectively) should not be surprised to be living in an improbable situation, if that is the sort of situation in which the observer was most likely to have existed.
[Synthesis] The problem with improbable situations is that they are also likely to be ephemeral. Let us leave aside the question of whether the physical constants can change over time in such a way as to make life impossible (it is not completely certain that they cannot). More prosaically, it is possible to look on the evolution of the biosphere and of the human race within it as a series of one potentially lethal disaster after another, each survived by pure chance. The Anthropic Principle may explain why we observe such an unlikely world, but it offers no promise that this unlikely situation will continue. There are several versions of the Doomsday Argument, but they all seem to be reassertions of the Copernican Principle. In this context, that means that our unlikely world should turn into the more probable sort of world that has no people in it. Something they also all have in common, when they are given mathematical expression, is that they hint at Doom Soon.
John Leslie is a Professor Emeritus of philosophy at the University of Guelph, Ontario. He has written extensively on the philosophical implications of modern cosmology and on ethics, both of which are relevant to this study. Judging by this book, he is a theistic neoplatonist with a utilitarian theory of ethics that is objective without being Kantian or deontological. This particular combination is new to me, but it works surprisingly well. In any case, the version of the Doomsday Argument at issue here was derived from a suggestion by Brandon Carter, but it is in large measure Leslie's. Whereas Carter's own fully developed Argument treated of the probable amount of time remaining to the life of the biosphere (about 20 minutes, by some estimates), Leslie's approach applies Bayes's Rule to human population history. The result, as Leslie points out, is not so much a proof of Doom Soon as a way to recalculate the risks associated with potentially world-destroying activities.
The Carter-Leslie Doomsday Argument makes most sense if you make some factual assumptions. First, you should assume that the number of people who will have ever lived will be finite. (This is reasonable but not inevitable: there are cosmological theories which make an infinity of future human beings a possibility, or even a necessity.) Second, you should assume that human population tends to increase geometrically over time, so that a historical graph of world population produces a Malthusian slope. Third, you should assume that the world is totally or substantially deterministic, so that events in the past have reasonably reliable implications for what happens in the future.
Now, consider your own position in the history of the human race (which Leslie estimates for convenience as going back 100,000 years). If you are reading this about the end of the twentieth century, then about 10% of all the people who have ever lived are alive with you. This position is unique, since never before has such a large fraction of historical humanity been alive at once. What implications does your unusual position in history have for the length of history as a whole? Because you know that some fraction of all the people who will ever be born have already either died or are still living, you know that you are closer to the end of the species than was any previous generation, but how much closer?
Imagine two possible human futures. One is a Grand Galactic Future, in which the species (or conscious entities of its devising) colonize space over millions of years. In that case, you will have been born among the earliest, tiniest fraction of all the trillions of people who will have ever lived. That would make your position not just unique but very surprising indeed, since you would have expected to have been born among all those future trillions. Now imagine a second future, in which the human race is extinct by AD 2150 (a figure that Leslie picks out of the air, unless I missed something). In that case, your position among the 10% of historical humanity alive circa AD 2000 makes perfect sense. The reason is the Anthropic Principle: all observers, yourself included, could expect to have been born into one of the later, larger generations rather than in one of the smaller, earlier ones.
If you live in a Doom Soon generation, of course, then you live in an unusual position in time, contrary to what the Copernican Principle would lead you to expect. The same would be true if we are only in the opening moments of the Grand Galactic Future. The difference is that the latter hypothesis would gain no support from the Anthropic Principle, since it assumes that most observers will live far in the future. As a matter of probabilities, then, you should prefer the hypothesis of Doom Soon.
This is a very annoying conclusion, not least because it is actually much more defensible than it looks. We do in fact use reasoning similar to the Carter-Leslie Doomsday Argument all the time. Bull markets in stocks, for instance, while they do not progress as evenly as a Malthusian population graph, do show an inverse correlation between prices and the length of time the bull market has still to run. If humanity were a mutual fund, the wise investor would think about selling around now.
There are, of course, numerous objections to Doom Soon, and Leslie discusses them at great length and in appalling detail. (There are also versions of the Anthopic Principle that require the Grand Galactic Future, but they are another story.) The chief objection, perhaps, is that anyone living at any point in the past could have invoked Doom Soon. A Roman could have remarked how unlikely it would be if he were living in an early generation that made up only a tiny fraction of all the people who would ever have lived. Leslie says that this earlier person would have been right: his position in time would have been unlikely, but it would also have been early. He says the population explosion of the last two centuries substantially changes the odds that we would be justified in making the same surmise. Well, maybe, but the actual graph of world population has not been nearly so regular as Malthus could have wished.
"Changing the odds" is the operative point of the book. Leslie does not think that Doom Soon is inevitable, and in fact he has high hopes for the Grand Galactic Future. The purpose of the exercise is to suggest that dangers to the future of humanity that seem very speculative if you think a long future nearly certain become much more menacing if you suspect that a short future has a substantial probability. This would suggest, therefore, that we might be well-advised to react to some threats more vigorously than we would if we thought that human extinction in the near future were no more likely than it had been at any point in the past.
Leslie illustrates how much more worried we should be by using Bayes's Rule, which is a mechanism for recalculating the initial probability of a hypothesis in the light of later evidence. You might, for instance, think it 98% likely that 1000 people had entered a lottery in which you are participating, but only 2% likely that just 10 people had entered. Suppose, though, that your ticket was drawn from the lottery urn on the first three tries. You would then have good reason to suppose that the guess of 10 participants was 67% likely. Similarly, assuming that the total number of people who will ever be born is finite, the fact that you have been born so early in the species' history suggests that the total number of people who will ever be born is not astronomical. In the same way, the probabilities of specific risks to the survival of the human race, which we might be inclined to dismiss at 1% or 2%, would undergo inflation comparable to that of the probability of the lottery urn having had just 10 tickets in it. The argument for risk inflation stands even though the risks from the perils in question are not quantifiable.
Be this as it may, much of the book is a compilation of such perils. As the first publisher's rejection letter for the Book of Revelation said, most of this is derivative. We glance briefly at the people's beloved asteroids, sweat through another explanation of the Greenhouse Effect and take the pulse of current prospects for a universal killer plague. The most intriguing of the technological dangers to humanity's longevity is based on the suggestion that our universe could conceivably have a "false vacuum." In that case, experiments in high temperature physics might collapse the metastability of the quantum mechanical fool's paradise in which we have been living. This would end not just the world, but the universe. Explaining the physics behind this "danger" would spoil the fun, since it is pretty clear that the danger is chimerical. Still, it alarms some people enough that they picket Fermilab at Batavia, Illinois, to demand the closure of a particle accelerator there.
By far the most valuable and readable part of "The End of the World" is its treatment of the ethics of human survival. Even if the human race's near-term extinction is much less than 50% likely, survival could still require an affirmative decision on the part of human leaders to ensure that the human race has a long future. Leslie makes a good case that the predominant theories of ethics today offer no rational grounds for such a decision. John Rawls's "A Theory of Justice" comes in for particular attack. Rawls's confining ethical imperatives to "fairness," says Leslie, creates no particular imperative to improve people's overall well-being, it simply requires that they be treated equally. More to the point, it does not imply that, all things being equal, large populations might be better than small populations, or even better than zero population. Favorable contemplation of the latter possibility is not confined to "deep ecologists," by the way. There are ethicists who argue that, since there will inevitably be unhappy people in any human population, it would be better if there were no such populations. As my grammar school principal used to say, it's just a few who spoil it for all the rest.
A decision to preserve the human race could be sharp and dramatic, like whether to launch a nuclear war, or reply to a first strike with a second. It could involve immediate costs for a long-term payoff, like instituting economic restructuring to minimize the artificial Greenhouse Effect. On the other hand, it could be something as mundane as demographic policy. Leslie points out that the human race would become extinct by AD 2400 if it had Germany's birthrate. It is not inconceivable that, in the next century, the world will have such a rate.
I will not disguise from the reader that this book is a bit of a mess. The doomsday scenarios are piled on without much attention to order or plausibility. I myself might have voted to end the world, if I had had to read one more illustration of Bayes's Rule using an urn and lottery. Still, it is rare to come across a book that makes such far-reaching connections. For instance, the question of whether we should ensure the existence of possibly less-than-blissful human societies in the future is shown to bear on the question of how a good God could have created a less-than-perfect world. Also, I don't think I have ever seen a more intriguing argument against Hume's theory of ethics than the one Leslie outlines.
Altogether, "The End of the World" is a valuable exercise. One hopes that similar Doom Soon books will continue to be published for a very long time to come.
Copyright © 1998 by John J. Reilly
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