A reader sent me a link to yet another purported Bayesian argument for the existence of god, this time by a physicist named Stephen Unwin. It’s actually very similar to Swinburne’s argument, which I discussed back at the old home of this blog. The difference is the degree of *dishonesty* demonstrated by the author.
As usual, you can only see the entire argument if you buy his book. But from a number of reviews of the book, and a self-interview posted on his personal website, we can get the gist. Scientific American’s review has the best concise description of his argument that I could find: (the equation in it is retyped by me.)
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:
Pafter = Pbefore×D/(Pbefore×D + 100% -Pbefore)
The probability of God’s existence after the evidence is considered is afunction of the probability before times D (“Divine Indicator Scale”): 10 indicates the evidence is 10 times as likely to be produced if God exists, 2 is two times as likely if God exists, 1 is neutral, 0.5 is moderately more likely if God does not exist, and 0.1 is much more likely if God does not exist. Unwin offers the following figures for six lines of evidence: recognition of goodness (D = 10), existence of moral evil (D = 0.5), existence of natural evil (D = 0.1), intranatural miracles (prayers) (D = 2), extranatural miracles (resurrection) (D = 1), and religious experiences (D = 2).
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.”
It’s pretty clear looking at this that the argument is nothing more than “I assert God exists, therefore God exists”. The “probability” result is generated by pulling numbers at random for his D-value. Even he admits that the numbers are “subjective”, but I would go much further than that: the numbers are fundamentally built on the assumption of the existence of god. How can you pretend that you haven’t already accepted the assumption that god exists, and then use stories about the occurrence of divine interventions as facts?
But this doesn’t touch on the reason that I call him dishonest. So far, it’s just sloppiness; typical of the sloppy reasoning of religious people trying to make arguments for the existence of god. But then, on his website, there’s a little self-interview:
Q: So does He exist?
SDU: I don’t know. Although my book does expand on this response.
It goes on like that. He claims to not know; to not have a belief about whether or not there is a god; that his book is an honest enquiry by someone uncertain, trying to use evidence to reason about whether or not god exists.
He’s lying. Plain and simple. Everything about his argument is completely predicated on his acceptance of the existence of god. And there’s no way that he’s dumb enough to not know that. But the argument seems so much more convincing to a layman if the author isn’t sure, but is just carefully working through the probabilities. And that final figure: exactly 2/3s… It’s nicely convenient. After all, he’s not saying he’s sure; but he’s saying that an objective review of the evidence gives a number that makes it look good, while not certain – it preserves that illusion of objectivity.
This guy is using his scientific background to give him authority as someone who understands how this kind of math works; and then he’s lying about his intentions in order to increase the credibility of his argument.