The Probability of God by Stephen D. Unwin

If John Derbyshire picked a difficult task (attempting to explain the ?World?s Greatest Unsolved Math Problem?) then Stephen Unwin has chose a Herculean one. Unwin, a physicist and risk management consultant and author of The Probability of God, set as his objective ?to calculate the numerical probability that God exists.? Not satisfied with that task he also hopes to ?determine the relationship between this probability and the notion of religious faith.? To attempt to accomplish all this in less than 250 pages is no mean feat.

The question is of course is did he do it? Well, yes and no. He certainly produces a number and he discusses the relationship between this number and faith. Along the way he shows his work and explains each step, but whether you accept his answer depends heavily on whether you approve of his process. In the end, I think Unwin outlines a rational process whereby one can attempt to attribute a probability to the proposition that God exists. He ends up with a number that is north of 50% but getting there is not simple or with controversy. In playing out this unique experiments, however, he provides useful insights into math, science, and faith. This may not be “proving” God?s existence but it is valuable nonetheless.


In order to proceed with his proposition intelligently Unwin must first define some terms. The first is the most important one, ?God.? What exactly does he mean by God? Unwin does not get into complicated theology. Instead, he simply narrows the definition down to the ?person-God? of the major monotheistic faiths (Judaism, Christianity, and Islam). Unwin?s God is a personal and interactive God not simply a prime mover or a pantheistic god of nature. He also takes a little side trip to discuss another controversial attempt to make belief in God rational, Intelligent Design. Unwin, however, rejects the current Watchmaker theory arguments based on the anthropic principle; i.e. only an ordered universe can be observed.

Having tackled what he means by God, Unwin moves onto to explicate what he means by probability. The simply answer is Bayes Theorem. He distinguishes Bayesian approach to probability from a frequentist one. The frequentist approach is best described by the coin flip. From this perspective we expect the coin to land heads 50% of the time so its probability is 50%. Roll a six-sided die and the probability of any side coming up is 16.7 or one-sixth. In contrast, Unwin asserts that in ?the Bayesian world, a probability is an expression of a degree of belief.? Using his background in quantum physics, he uses the following example:

When the statement is made by the physicist that there is a 5 percent probability neutrinos are massless, she means that on a scale of 0 to 100, her degree of belief that the said particles are massless is 5.

Unwin does a good job of laying out the Bayesian approach and any attempt on my part to replicate that explanation would be redundant and inelegant. The basic idea, however, is to take the evidence one has and to convert the ?data? into a mathematical formula and thus a probability. Like the scientist with the neutrinos, Unwin attempts to arrive at a numerical probability. Instead of sub-atomic particles, the subject is God?s existence.

Unwin starts with the proposition that he is neutral on God?s existence. In other words the starting point is 50/50 on God?s existence. This is the a priori probability from a neutral perspective but he goes further. To inform his formula Unwin uses six evidentiary areas that affect the probability that God exists. They are:
1. The recognition of goodness.
2. The existence of moral evil.
3. The existence of natural evil.
4. Intra-natural miracles.
5. Extra-natural miracles.
6. Religious experiences.
As you can see from the titles, these are common areas of disagreement but oft-sited vindication for the existence, or lack thereof, of God. Unwin goes through each evidentiary area and assigns it a numeric factor or scale that he can then plug into his formula. The more a area increases the probability that God exists the higher the factor. The factors are limited to 10 (much more likely), 2 (moderately more likely), 1 (neutral), ½ (moderately more unlikely), and 1/10 (much more unlikely). Again, I don?t want to spoil the meat of the book so I won?t outline Unwin?s math but what is useful is his honesty. He straightforwardly and pragmatically outlines his thoughts on these subjects. You might not agree with him but you can follow his arguments and his math. In fact, as an appendix Unwin provides a simple way to plug the formula into a spreadsheet so you can do the math yourself, experimenting with the scale according to your own beliefs.

Without spoiling the conclusion, I can tell you that Unwin places the probability that God exists north of the 50% he started with. The end result is that Unwin thinks that belief in God is an eminently reasonable position given the evidence. To further reinforce this view, he takes a look at Pascal?s Wager and probabilistic decision theory in light of both his newfound probability and the logic of math. Unwin uses this as a stepping-stone to a discussion on faith itself. He ponders the definition of faith. Is faith a leap in the dark from the end of logic and rationality to a belief in God? Is it simply an extension of ones rational belief? It is a complicated issue and Unwin provides a fascinating discussion. In the end, he compares faith and reason as complimentary but not wholly separate things:

faith reflects an experiential belief rather than a reasoned belief in a mundane proposition of statistics, logic, or concrete fact. Perhaps probability can be viewed as a snapshot taken in the cold light of logic, whereas faith is more like the exhilaration of experiencing great music. Yet, as we discussed, faith can be attached to the proposition with which the more conventional meaning of ?true? (or ?false?) can be associated.

Unwin uses a formula to explain: Belief in God = the probability of God + Faith in God. He asserts that a proper balance is necessary for faith to have meaning. If faith simply picks up the tab for whatever reason can?t cover then the more you know the less faith you have. This would put reason at odds with faith and vice versa. Unwin believes that humans use both the cold logic of probability and the more existential trust to combine in a belief in God. In this way, belief in God?s existence is rational even though God can?t be proven by mathematical formula and yet faith is still an integral part of that belief.

If this subject interests you I would recommend you read the book for yourself. Unwin?s tone is lighthearted and jovial with being mocking. He covers issues of math, science, and faith in a way that the laymen can understand and yet the ideas remain challenging. I doubt that this book will, in and of itself, convince anyone that God exists who previously discounted that possibility. But it does provide and interesting and thought provoking way of looking at the issues involved. I think that he makes a solid case for the rationality of a belief in God. In a climate where religion is often mocked as irrational or even psychotic, this is a valuable accomplishment. People will be hard pressed to write Unwin off as a religious fanatic or unscientific rube. Nor can they attack him as a closed minded fundamentalist given that he avoids most controversial theological issues. You might disagree with his ideas and assumptions but you will have to deal with them head on.

If you have an interest in issues of faith and science, or simply like to explore ideas, I recommend you read the book yourself. It is an easy and interesting read and less than 250 pages to boot. It is not everyday you have the opportunity to learn the basics of an important and useful mathematical subject, explore the nature of faith, and stretch your imagination at the same time.

About the author

Kevin Holtsberry

I work in communications and public affairs. I try to squeeze in as much reading as I can while still spending time with my wife and two kids (and cheering on the Pittsburgh Steelers and Michigan Wolverines during football season - oh, and watching golf too).

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  • fao. Dr. Stephan D. Unwin, Ohio

    Dera Sir, with due respect I feel compelled to point out that in my view the formular of >Probability of God must include the existance of >Primary Reality and our inability to comprehend >Ineffabilis Deus

    Man, reaching back now 13 billion of years or more nearly to the creation of his >Secondary Reality is realizing that our world and the whole universe are only in our brains – however gifted.

    And we are learning that our >Secondary Reality and our whole being in space and time consists of infinitesimally small positive and negative field distortions – adding up to nil.

    I contend, the foremost question of all is not the >Probability of God. The >Primary Reality does exist !

    We should ask and evaluate:
    Does God take interest in this universe He created ?
    Dooes God take interest in the human race He created ?
    Does God take interest in the individual being ?
    Does God interfere or intervene in our history ?
    Does God want any definite behaviour or believe ?

    Has God spoken to mankind at any time and in any way ?

    We can calculate but never prove or disprove the fundamental questions. If the result of the formular would be beyond any doubt Zero or 100% then there would be no room left for our >free will !

    Yours sincerely

    Gerhard Dombrowsky
    Erftstrasse 9
    D-41564 Kaarst
    Germany