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The Probability of God: A Simple Calculation That Proves the Ultimate Truth (英語) ペーパーバック – 2004/10/26
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Does God exist?
This is probably the most debated question in the history of mankind. Scholars, scientists, and philosophers have spent their lifetimes trying to prove or disprove the existence of God, only to have their theories crucified by other scholars, scientists, and philosophers. Where the debate breaks down is in the ambiguities and colloquialisms of language. But, by using a universal, unambiguous language—namely, mathematics—can this question finally be answered definitively? That’s what Dr. Stephen Unwin attempts to do in this riveting, accessible, and witty book, The Probability of God.
At its core, this groundbreaking book reveals how a math equation developed more than 200 years ago by noted European philosopher Thomas Bayes can be used to calculate the probability that God exists. The equation itself is much more complicated than a simple coin toss (heads, He’s up there running the show; tails, He’s not). Yet Dr. Unwin writes with a clarity that makes his mathematical proof easy for even the nonmathematician to understand and a verve that makes his book a delight to read. Leading you carefully through each step in his argument, he demonstrates in the end that God does indeed exist.
Whether you’re a devout believer and agree with Dr. Unwin’s proof or are unsure about all things divine, you will find this provocative book enlightening and engaging.
“One of the most innovative works [in the science and religion movement] is The Probability of God...An entertaining exercise in thinking.”—Michael Shermer, Scientific American
“Unwin’s book [is] peppered with wry, self-deprecating humor that makes the scienti?c discussions more accessible...Spiritually inspiring.”--Chicago Sun Times
“A pleasantly breezy account of some complicated matters well worth learning about.”--Philadelphia Inquirer
“One of the best things about the book is its humor.”--Cleveland Plain Dealer
“In a book that is surprisingly lighthearted and funny, Unwin manages to pack in a lot of facts about science and philosophy.”--Salt Lake Tribune
"Unwin's book, which is peppered with wry, self-deprecating humor that makes the scientific discussions more accessible, may prove spiritually inspiring."—Chicago Sun Times
"A pleasantly breezy account of some complicated matters well worth learning about."–Philadelphia Inquirer
"Stephen Unwin has pulled off the seemingly impossible—a profound and instructive discussion of God delivered in an entertaining, witty and no-nonsense style. His quirky prose conceals a wealth of information about science, religion and the grounds for rational belief. He bypasses the indulgent obscurantism so characteristic of contemporary theological writing and goes straight to the heart of the matter: Is belief in God a reasonable deduction given the evidence? That Unwin can achieve this with minimal mathematics is a testimony to his incisive mind and impressive writing skills."
—Paul Davies, Templeton Prize winner and author of The Mind of God and The Cosmic Blueprint
"This book is very bad news for anyone planning a career in Evil. Engaging, witty, concise and clear, Dr. Unwin's book achieves two impossible things: it makes Theology and Probability Theory accessible to humans."
—Rob Grant, co-creator of the "Red Dwarf" television series and author of Backwards
"The Probability of God is witty, it's fun to read--you keep wanting to goon, chapter after chapter, and keep going to figure out this probability. Ifyou're looking for a book that's intellectually stimulating, kind of fun,and compelling-- this is it."
"Yes, the book has humor, a trait that math sorely needs... I especially liked the way he mixed humor and scientific rigor."
"Stephen Unwin has boldly gone where no theoretical physicist has gone before."
-Dayton Daily News
"Unwin did not set out to prove one way or another the existence of God. He just wanted to figure the odds."
-Australian Herald Sun
There are some flaws, however. He states very briefly (on pg 95) that he is assuming that the evidentiary areas are not correlated, and then he never returns to the issue. I think that this is probably pretty important, and I would have appreciated at least an appendix on this subject for the more mathematically muscular -- perhaps instead of the muddled discussion of faith and belief. It appears to me that this is important because I believe that treating correlations correctly is the mechanism by which one avoids "cooking" the analysis by introducing a number of basically equivalent pieces of evidence that differ only in how they are stated. If these equivalent evidentiary areas all point in one direction but if they are treated as if they were independent, I think that this will yield a grossly erroneous result.
That makes me want to understand just how independent the evidentiary areas he presents are, and although I have a feeling that he did a pretty good job of choosing them fairly, I don't know how to test this. I also don't know how to test whether additional evidentiary areas I might make up are fair in this sense.
More accurately, it formulates the existence of God as something that we may not be entirely certain of, and therefore depends on vague estimates of probabilities, which are subjective measures of one's predilections to believe arguments of one type or another. Thus, this book acknowledges the subjective character of the question, and instead of focusing too much on the AUTHOR'S probability of the existence of God, it provides the reader with the tools to calculate the READER'S probabiliity of the existence of God.
This is a perfect example of a situation where the usual notion of probability often taught (when you flip the coin a thousand times it comes up heads as often as it comes up tails, for instance) comes up short. This notion makes no sense when it comes to matters of fact, which cannot be repeated in experiment. For this, Unwin describes the Bayesian interpretation of probability, pioneered in the early 1900s, where probabilities measure rational belief.
Unwin's work here applies the Bayesian notion of probability to five classical arguments used in the debate over the existence of God. As such, he has added something new and interesting to the debate.
In execution, however, there are problems and fallacious arguments--a feature that can be beneficial in a classroom where these deficiencies can be debated and discussed. First, Unwin's choice of five arguments determines the resulting probability more than he would like to admit. If some arguments had been subdivided into subarguments, the probability would change, giving that argument more weight. Similarly, there are fairly abstract philosophical arguments that were not brought in at all, which may be good for most people who are suspicious of such abstract arguments (perhaps with good reason) but it is important to recognize that not all arguments for or against the existence of God are considered.
Furthermore, the most difficult issue of all problems with the bayesian approach to probability, the initial a priori probability, he skips over facilely by declaring it to be 1/2. This may perhaps be better defended than any other number, but the explanation here is lacking.
Unwin also has a rating system to deal with the effect of each evidence area on the overall probability of the existence of God, that is very coarse, as it must be in such situations (can you imagine anyone arguing that the existence of evil in the world, given that God exists, is 23% as opposed to 24%, for instance?) The representative percent probabilities (1/11, 1/3, 1/2, 2/3, 10/11) he gets is fairly influential over the resulting answer, more than Unwin would like to admit. He certanly doesn't get two decimal places of accuracy, as he claims.
Now there is structure to bayesian analysis that Unwin does not discuss. In a chain of evidence, evidence that favors a hypothesis cancels evidence that works against that hypothesis in a very precise way, so that given his choices of probabilities, he is bound to get fairly moderate numbers (especially since he made sure to have arguments that favor the existence of God and arguments against).
Now, beyond the math, I should also mention a few other criticisms:
1. Philosophers and theologians have brought a great deal of nuance to these arguments that Unwin does not acknowledge. To take one example, the problem of evil in the world is not necessarily stacked against a traditional theist as it at first appears, depending on to what extent you accept various explanations offered by theists.
2. As in any philosophical work, there are arguments Unwin makes that would not work for everyone. For instance, his use of the anthropic principle to explain the fruitfulness of our universe depends on the many-worlds interpretation of quantum mechanics, which from a scientific methodological viewpoint, is as metaphysical and unmotivated as the existence of God in the first place. Still, the fact that Unwin does not focus on the ANSWER but on the PROCESS the reader can use to find his or her own answer, ameliorates this consideration considerably, as long as the reader goes through this exercise.
3. Unwin compares two positions: the traditional monotheist position of the Abrahamic faiths against the materialistic version of modern atheism. Comparing many positions at once would make this story much more complicated, so it's easy to see why he didn't bother. But for many people, the competing theories are of a different kind.
5. The most compelling reasons for belief, for many people, often involve issues beyond rationality, such as personal encounters with the divine, or the influence of a community of belief. And it is not at all clear that these reasons are "bad" reasons that should be shunned in these considerations. But they are excluded from the start in Unwin's work.
Still, the fact that Unwin seeks to provide tools, not answer questions, makes this book valuable as a *beginning* of a conversation.
His description of faith as the extra-rational piece that goes beyond reason falls short of traditional understandings of faith. Faith is not the magical extra extent to which one believes something that one would not believe otherwise. It is a trust that one chooses to make when reason is there: "faith is a leap into the light, not a leap into darkness". Even if this were the case, it is hardly acceptable to posit faith and rationality as adding together--as if a particularly gullible individual who has very good reason to believe something could end up with a probability of higher than 100%! Yet this is the model he argues for, somewhat unconvincingly.
With all these disagreements aside, I recommend the book, not as a source of answers, but a way to start thinking about these questions for yourself, perhaps leading you to write another book that reflects your perspective on this question. After reading this book, I was upset at the problems I mentioned above, but decided to organize a group of students to read this together, not in spite of the problems, but perhaps because of them. It is a positive thing for discussions to be deeper, especially when one does not agree with the other side.
Beyond all of this, Unwin is a very clear writer, and explains concepts of probability, risk, and Bayesian analysis so that anyone can understand it. He is also chatty and pleasant to read, with a sense of dry wit that can be delightful or annoying, depending on your personal taste in humor.
Overall, I recommend this book highly, as a beginning, not an end, to discussion.