# How probable is God?

Skeptic Michael Shermer has an article up over at Scientific American.com on Stephen D. Unwin’s book The Probability of God: A Simple Calculation That Proves the Ultimate Truth in which he discusses how the risk management consultant out of Ohio came to the conclusion that there’s a 67% probability that God exists.

God’s Number Is Up – Scientific American.com

Unwin rejects most scientific attempts to prove the divine—such as the anthropic principle and intelligent design—concluding that this “is not the sort of evidence that points in either direction, for or against.” Instead he employs Bayesian probabilities, a statistical method devised by 18th-century Presbyterian minister and mathematician Reverend Thomas Bayes. Unwin begins with a 50 percent probability that God exists (because 50–50 represents “maximum ignorance”), then applies a modified Bayesian theorem.

Shermer provides a brief explanation of the formula Unwin came up with and which numbers he plugged into it and then points out the glaringly obvious reason why this little exercise is completely pointless. I’ve boldfaced the glaring error for those visitors who are a little slow on the uptake.

Plugging these figures into the above formula (in sequence, where the Pafter figure for the first computation is used for the Pbefore figure in the second computation, and so on for all six Ds), Unwin concludes: “The probability that God exists is 67%.” Remarkably, he then confesses: “This number has a subjective element since it reflects my assessment of the evidence. It isn’t as if we have calculated the value of pi for the first time.”

Shermer goes on to recalculate the formula using his own assessment of the evidence as based on his theories of “the evolutionary origins of morality and the sociocultural foundation of religious beliefs and faith” and comes up with only a 2% probability of God’s existence.

This surprising admission on Unwin’s part that the numbers he chose for the calculation are completely subjective has in no way dampened the exposure this book has received throughout the news media and more than a few True Believers™ have sent me links to the book in hopes of showing me how wrong I am to be an atheist. Truth is, there’s nothing in Unwin’s formula or book that’s likely to convince anyone who isn’t already a believer that he’s got it right. Still, there’s plenty of people out there who are making lots of money selling books that claim to offer scientific proof of God which will continue to be popular with the folks who have trouble accepting the idea of God purely on faith.

I’ve had my own brush with this sort of thing in the past back when I was working as a Desktop Publishing Consultant for a local Kinko’s Copies store. There was this local fellow in his late 60’s or early 70’s who’d come into the store from time to time to have me make custom business cards and brochures for him. This fellow needed these things in order to publicize his discovery of the proof of God via a very complex mathematical formula he had concocted that was printed on the back of his business cards and was the focus of the overview in the brochures. Every time he’d come in to have me do some work he’d wait patiently while I added whatever he wanted to the cards or brochures and he’d explain, every time, in great detail, just what each X or Y on the formula stood for and how when you plugged in all the numbers and calculated it out it ended up equaling the number 1, which is God according to this guy, and therefore is proof that God exists. Some of the numbers were things such as the speed of light or the acceleration of gravity or some other well known scientific number and others were, as near as I could tell, complete arbitrary assignments he pulled out of his ass, but when you worked it all out via his formula it did come out to the number 1.

He was quite proud of this bit of mathematical mumbo jumbo and yet very concerned that not many people took him seriously on it. Well known scientists in particular, he’d often lament, would rarely respond to his letters describing his discovery and the few that did had left him feeling as though he was being humored. I was guilty of humoring him myself for the simple reason that I wasn’t in a position that allowed me to engage him in a discussion on the issue without risking my employment. Even if I had been I don’t know that I would have bothered as although he was a bit of a loon in this one regard, he was largely pleasant and harmless. With the level of belief he had invested in his idea and at the age he was all I would have accomplished in challenging him on it would’ve been to piss him off so there wasn’t much point. He was a classic victim of the Henrietta Syndrome.

## 6 thoughts on “How probable is God?”

1. Well, if it were that easy to prove it, anyone could.

(Besides, 67% doesn’t actually “prove” anything, or disprove it, either.)

cap: standard

2. If you apply a formula such as Unwin’s to alien life forms the probability that such life exists must approach somewhere around 90% or greater.  Yet, most Christians don’t believe life exists outside of this planet because they have seen no evidence of such life and there is no mention in the bible of such life.  It’s another ruse of pick and chose.

3. Not surprisingly Bayes’ theorem can work very well when it is properly applied to a physical problem. Here is an example from a 1968 paper by Edwin James of Washington University.

For example—a famous example that Laplace actually did solve—proposition A might be the statement that the unknown mass of Saturn Ms lies in a specfied interval, B the data from observations about the mutual perturbations of Jupiter and Saturn, C the common sense observation that Ms cannot be so small that Saturn would lose its rings; or so large that Saturn would disrupt the solar system. Laplace reported that the data up to the end of the 18th century, Bayes’ theorem estimates Ms to be (1/3512) of the solar mass, and gives a probability of .99991, or odds of 11,000:1, that Ms lies within 1% of that value. Another 150 years accumulation of data has raised the estimate 0.63 percent.

He also used Bayes’ theorem to decide which astronomy problems to work on.

Laplace also published (1812) a remarkable two—volume treatise on probability theory in which the analytical techniques for Bayseian calculations were developed to a level that is seldom surpassed today.

Unfortunately, there were some conceptual difficulties with the math that Laplace developed that he didn’t explain clearly enough, and Bayseian analysis languished until the middle of the last century.

Two other notably successful applications that I am aware of occurred in the 1960s when a Navy Scientist used Bayesian techniques to locate the H-Bomb that the Air Force carelessly dropped in the ocean and to locate the USS Scorpion.

4. I just finished this book. Unwin does do a good job of explaining Bayesian analysis (which, as VernR explained, works quite well for some problems), but fails to defend its use for a gauzy problem like the existence of God.

One huge problem I had with his analysis was his prior probability of 50%. I wouldn’t start at 50% for the existence of unicorns or dragons, so I started at 10% (to be generous), added the evidentiary area of “no direct sensory evidence exists for God’s existence,” worked through the steps, and came up with a probability of 0.01%.

Bottom line: this analysis will only reflect what you think already, which makes it worthless. I’m sure a devout Southern Baptist would end up with a probability of 99.9%.

5. This will probably seem long-winded, but I will finally get around to saying that both Les and KT are right.

Over the years I have tried to understand how to apply Baye’s theorem and have never succeeded very well—meaning that I can work simple textbook examples, if I don’t have to close the textbook. When I read through the original post and the SA article, two things threw me off of what was going on in this particular problem: the ‘modified’ form of the Bayes equation which I had never seen before I read Shermer’s article, and Unwin’s Divinity Scale which I didn’t immediately recognize as a scale of odds ratios (even though it was pretty clearly labeled as such).

The light-bulb flickered on earlier today when I realized that Unwin’s methodology was similar to a pedagogical exercise in sequential testing that Jaynes presented in one of his papers*. Jaynes was originally a EE so he re-expressed odds ratios on a decibel scale**. If you plot probability level (y axis) vs e, with e values from say -30 to + 30, you get a nice s-curve where y ranges values very nearly zero to y values very nearly one. At the y-axis crossing (e = 0) the p-level is one half.

In his analysis Unwin takes a reasonable Baysean position that, since he doesn’t know whether God exists or not, his prior assessment is that p = ½ (e = 0). Because of the transformation, what was a multiplicative (updating) process is now an additive process. Each piece of new evidence (D-value) results in a shift to the right or left along the e axis. Unwin’s D-values result in the following e value shifts

+10, -3, -10, +3, 0, +3.

This sequence nets to an overall shift in e to a final value of 3, which corresponds to an odds ratio of 2:1 (a final Pafter equal to 2/3).

Well, that is a nice numerical exercise, but statistics and probability theory were developed to predict outcomes of physical processes, to tell how precisely we know something after taking a series of measurements, or to use data to make ‘good’ decisions (about processes). Someone choosing a value on Unwin’s D-scale is in no sense making a measurement. That’s why Les was right—D-scale values are totally subjective personal assessments.

This next gets to KT’s point. In his writings, Jaynes indicated that if analysts have prior knowledge, they should use that knowledge as a starting point for their subsequent analyses. I say that in a so called Baysean discussion about the existence of God that true believers (David and others) would use an initial e value of + infinity and a true non-believers (nowiser and others) would use an initial e value of – infinity. No finite sequence of D-values would result in final assessments differing from initial assessments.

Shermer gets the final word.

Religious faith depends on a host of social, psychological and emotional factors that have little or nothing to do with probabilities evidence and logic. This is faith’s inescapable weakness. It is also, undeniably, its greatest power.

* My latest text book
** e = 10 log(odds ratio).

6. VernR commented that:

This next gets to KT’s point. In his writings, Jaynes indicated that if analysts have prior knowledge, they should use that knowledge as a starting point for their subsequent analyses. I say that in a so called Baysean discussion about the existence of God that true believers (David and others) would use an initial e value of + infinity and a true non-believers (nowiser and others) would use an initial e value of – infinity. No finite sequence of D-values would result in final assessments differing from initial assessments.

My comment: While this may be true, Bayesian analysis is not useful if the prior rules out any hypothesis (as +/- infinity would do). A more honest approach would put a very small or very large value of e (depending on who you are) and then let the evidence fall where it may. It appears to me (not having read Unwin) that he is playing fast and loose with how one folds the data into the likelihood. The criticisms of his work that address this aspect are right on target.

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