The God Delusion
BAYESIAN ARGUMENTS
I think the oddest case I have seen attempted for the existence of God is the Bayesian argument recently put forward by Stephen Unwin in The Probability of God. I hesitated before including this argument, which is both weaker and less hallowed by antiquity than others. Unwin's book, however, received considerable journalistic attention when it was published in 2003, and it does give the opportunity to bring some explanatory threads together. I have some sympathy with his aims because, as argued in Chapter 2, I believe the existence of God as a scientific hypothesis is, at least in principle, investigable. Also, Unwin's quixotic attempt to put a number on the probability is quite agreeably funny.
The book's subtitle, A Simple Calculation that Proves the Ultimate Truth, has all the hallmarks of a late addition by the publisher, because such overweening confidence is not to be found in Unwin's text. The book is better seen as a 'How To' manual, a sort of Bayes' Theorem for Dummies, using the existence of God as a semi-facetious case study. Unwin could equally well have used a hypothetical murder as his test case to demonstrate Bayes' Theorem. The detective marshals the evidence. The fingerprints on the revolver point to Mrs Peacock. Quantify that suspicion by slapping a numerical likelihood on her. However, Professor Plum had a motive to frame her. Reduce the suspicion of Mrs Peacock by a corresponding numerical value. The forensic evidence suggests a 70 per cent likelihood that the revolver was fired accurately from a long distance, which argues for a culprit with military training. Quantify our raised suspicion of Colonel Mustard. The Reverend Green has the most plausible motive for murder.* Increase our numerical assessment of his likelihood. But the long blond hair on the victim's jacket could only belong to Miss Scarlet . . . and so on. A mix of more or less subjectively judged likelihoods churns around in the detective's mind, pulling him in different directions. Bayes' Theorem is supposed to help him to a conclusion. It is a mathematical engine for combining many estimated likelihoods and coming up with a final verdict, which bears its own quantitative estimate of likelihood. But of course that final estimate can only be as good as the original numbers fed in. These are usually subjectively judged, with all the doubts that inevitably flow from that. The GIGO principle (Garbage In, Garbage Out) is applicable here - and, in the case of Unwin's God example, applicable is too mild a word.
* The Reverend Green is the character's name in the versions of Cluedo sold in Britain (where the game originated), Australia, New Zealand, India and all other English-speaking areas except North America, where he suddenly becomes Mr Green. What is that all about?
Unwin is a risk management consultant who carries a torch for Bayesian inference, as against rival statistical methods. He illustrates Bayes' Theorem by taking on, not a murder, but the biggest test case of all, the existence of God. The plan is to start with complete uncertainty, which he chooses to quantify by assigning the existence and non-existence of God a 50 per cent starting likelihood each. Then he lists six facts that might bear on the matter, puts a numerical weighting on each, feeds the six numbers into the engine of Bayes' Theorem and sees what number pops out. The trouble is that (to repeat) the six weightings are not measured quantities but simply Stephen Unwin's own personal judgements, turned into numbers for the sake of the exercise. The six facts are:
1. We have a sense of goodness.
2. People do evil things (Hitler, Stalin, Saddam Hussein).
3. Nature does evil things (earthquakes, tsunamis, hurricanes).
4. There might be minor miracles (I lost my keys and found them again).
5. There might be major miracles (Jesus might have risen from the dead).
6. People have religious experiences.
For what it is worth (nothing, in my opinion), at the end of a ding-dong Bayesian race in which God surges ahead in the betting, then drops way back, then claws his way up to the 50 per cent mark from which he started, he finally ends up enjoying, in Unwin's estimation, a 67 per cent likelihood of existing. Unwin then decides that his Bayesian verdict of 67 per cent isn't high enough, so he takes the bizarre step of boosting it to 95 per cent by an emergency injection of 'faith'. It sounds like a joke, but that really is how he proceeds. I wish I could say how he justifies it, but there really is nothing to say. I have met this kind of absurdity elsewhere, when I have challenged religious but otherwise intelligent scientists to justify their belief, given their admission that there is no evidence: 'I admit that there's no evidence. There's a reason why it's called faith' (this last sentence uttered with almost truculent conviction, and no hint of apology or defensiveness).
Surprisingly, Unwin's list of six statements does not include the argument from design, nor any of Aquinas' five 'proofs', nor any of the various ontological arguments. He has no truck with them: they don't contribute even a minor fillip to his numerical estimate of God's likelihood. He discusses them and, as a good statistician, dismisses them as empty. I think this is to his credit, although his reason for discounting the design argument is different from mine. But the arguments that he does admit through his Bayesian door are, it seems to me, just as weak. That is only to say that the subjective likelihood weightings I would give to them are different from his, and who cares about subjective judgements anyway? He thinks the fact that we have a sense of right and wrong counts strongly in God's favour, whereas I don't see that it should really shift him, in either direction, from his initial prior expectation. Chapters 6 and 7 will show that there is no good case to be made for our possession of a sense of right and wrong having any clear connection with the existence of a supernatural deity. As in the case of our ability to appreciate a Beethoven quartet, our sense of goodness (though not necessarily our inducement to follow it) would be the way it is with a God and without a God.
On the other hand, Unwin thinks the existence of evil, especially natural catastrophes such as earthquakes and tsunamis, counts strongly against the likelihood that God exists. Here, Unwin's judgement is opposite to mine but goes along with many uncomfortable theologians. 'Theodicy' (the vindication of divine providence in the face of the existence of evil) keeps theologians awake at night. The authoritative Oxford Companion to Philosophy gives the problem of evil as 'the most powerful objection to traditional theism'. But it is an argument only against the existence of a good God. Goodness is no part of the definition of the God Hypothesis, merely a desirable add-on.
Admittedly, people of a theological bent are often chronically incapable of distinguishing what is true from what they'd like to be true. But, for a more sophisticated believer in some kind of supernatural intelligence, it is childishly easy to overcome the problem of evil. Simply postulate a nasty god - such as the one who stalks every page of the Old Testament. Or, if you don't like that, invent a separate evil god, call him Satan, and blame his cosmic battle against the good god for the evil in the world. Or - a more sophisticated solution - postulate a god with grander things to do than fuss about human distress. Or a god who is not indifferent to suffering but regards it as the price that has to be paid for free will in an orderly, lawful cosmos. Theologians can be found buying into all these rationalizations.
For these reasons, if I were redoing Unwin's Bayesian exercise, neither the problem of evil nor moral considerations in general would shift me far, one way or the other, from the null hypothesis (Unwin's 50 per cent). But I don't want to argue the point because, in any case, I can't get excited about personal opinions, whether Unwin's or mine.
There is a much more powerful argument, which does not depend upon subjective judgement, and it is the argument from improbability. It really does transport us dramatically away from 50 per cent agnosticism, far towards the extreme of theism in the view of many theists, far towards the extreme of atheism in my view. I have alluded to it several times already. The whole argument turns on the familiar question 'Who made God?', which most thinking people discover for themselves. A designer God cannot be used to explain organized complexity because any God capable of designing anything would have to be complex enough to demand the same kind of explanation in his own right. God presents an infinite regress from which he cannot help us to escape. This argument, as I shall show in the next chapter, demonstrates that God, though not technically disprovable, is very very improbable indeed.
4
WHY THERE ALMOST CERTAINLY IS NO GOD
The priests of the different religious sects . . . dread the advance of science as witches do the approach of daylight, and scowl on the fatal harbinger announcing the subdivision of the duperies on which they live.
— THOMAS JEFFERSON