In the realm of science and faith, I don’t have that much in common with Richard Dawkins, but I do agree with him on one thing: namely, that it is somewhat ridiculous trying to work out the probability that God exists using Bayesian analysis. Bayesian analysis is a technique of statistical inference named after an 18th-century Presbyterian minister, Thomas Bayes. Properly used, it is a very powerful technique for estimating how confident we should be in some belief given the evidence we have. For instance, Bayesian analysis is used all the time in clinical trials. The technique works by giving an initial percentage of certainty to some belief, e.g. 0% if you are certain the belief is false, 100% if you know it’s true, and somewhere in between if you’re not sure. If you are then presented with some evidence and know how likely that evidence would be if the belief happened to be true, the Bayes theorem tells you how to work out how likely the belief is given the evidence and one’s prior degree of certainty.

Now, although Bayesian analysis is extremely versatile, I agree with Dawkins that it would be foolish to use this technique to establish the degree of certainty with which we should believe in God’s existence. Dawkins pours scorn on an author who begins with a prior certainty of 50% that God exists, and then uses evidence such as the fact we have a sense of goodness, the existence of evil, the reports of miracles, etc to conclude that we should be 67% certain that God exists. Dawkins’ objection is that it is impossible to come up with precise numbers for the conditional probability of these facts if God happened to exist. 

But my reason for not using Bayesian analysis in this context is entirely different from Dawkins’. For Bayesian analysis works on the assumption that you are unsure about the belief in question. But in the case of God’s existence, if you happen to be certain that God exists based on some proof such as one of the proofs of St Thomas Aquinas, then no amount of evidence is going to change your certainty. It’s like in mathematics – once you know that two plus two is four, no amount of evidence is going to make you change your mind.

But although Dawkins entirely dismisses the Bayesian approach to God, he takes another probabilistic argument more seriously. The so-called argument from improbability is an argument put forward by the astronomer Fred Hoyle. He argued that the probability of life originating on Earth is no greater than the chance that a hurricane, sweeping through a scrapyard, would have the luck to assemble a Boeing 747. Although Hoyle was arguing against an evolutionary account of life’s emergence, some theists seized on Hoyle’s argument to conclude that an intelligent designer God must exist. Not to be undone, Dawkins attempts to turn the tables on these theists by saying that if life was designed by God, then God must be more complicated than life, and hence the existence of God must be even more improbable than the existence of life.

Dawkins’ argument for the improbability of God’s existence fails to take account of a fundamental property traditionally attributed to God, namely divine simplicity. As St Thomas Aquinas explains, divine simplicity means that every conceivable form of composition must be denied of God. Because this doctrine is just a denial of composition, we’re not supposed to have some positive and definite idea of what it’s like for God to be simple – the best we can do is draw on analogies. 

For example, a sign of true intelligence isn’t one’s capacity to have complicated thoughts, but to have simple thoughts, for the highly intelligent are able to see the simplicity underlying all the complexity. And God who is pure intelligence is able to understand everything there is to understand by simply knowing Himself in one simple act of knowing. Dawkins only demonstrates that it is improbable that a complicated God exists, but St Aquinas would go even further and set this probability to 0%.

But although probabilistic reasoning in the context of theology can often lead us astray, there’s more to it than that. For as Christians we don’t just believe in a God of the philosophers. We believe in a God who has revealed Himself in history, and we have evidence for what we believe. We have the evidence of scripture and the living tradition of the Church. 

This is evidence that reasonable people can weigh up. The apostles were so sure that Christ had risen, they would rather die than deny this. That’s strong evidence. And it is our responsibility to preserve the evidence of our Faith so that with the aid of reason and the grace of God, future generations may come to embrace the Truth.

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