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.
The End of the World:
The Science and Ethics of Human Extinction
by John Leslie
310 pages, $28:00 (Hardcover)
This book 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.