The design arguments that have become popular in the last few years have invoked some impressive claims about probability that have an authoritative air to them. Many people who share the intuition that “this all couldn’t have possibly have happened by chance” find these arguments quite compelling.
Robin Collins, in God, Design, and Fine Tuning, claims: The existence of the fine-tuning is not improbable under theism. The existence of the fine-tuning is very improbable under the atheistic single-universe hypothesis. And in his version of the design argument, Richard Swinburne argues that it is exceedingly unlikely that there would be a lawlike universe of matter composed of simple parts that could just happen by random chance. But such a universe is what we would expect to find if there were a designer God who values beauty, simplicity, and who wishes to create a challenging environment for his human creations.
In order to make use of the Bayesian probability calculus, which these arguments do, part of what figures into the equation is something called a person’s prior probabilities. In order to attach a probability to some outcome that is unknown, Bayes theorem requires that I attach some value to the probable outcomes as I see them. Bayesian calculations are subjective in this fashion. In the design arguments above, we are asked to agree that the likelihood that the universe could have come out like it is without God is very, very low. It doesn’t seem like such a thing could have happened by chance or without some purposeful plan in the hands of a powerful being, does it?
But our subjective sense of likelihood here is really all that we have. We don’t have any real distribution data concerning universes that would let us say that 95% of the time in cases we have studied, universes with stable carbon molecules were designed by God. And only in a tiny number of cases of the millions of universes we have studied do life favorable conditions happen by chance. If we had those numbers, then it would clear a lot of things up. But we have one universe—the one we live in. And each of us only has the confines of our own mind in which to make a call about the probability or improbability of a life friendly universe by chance. No doubt for many people, when they consider the possibility that all of the physical laws just happened to line up the way they did by chance, or God did it, they find the latter much more likely. See my earlier post, Bogus Probability Judgments and God for an analysis of the false dilemma that is getting smuggled past us here.
In this sort of case, though, assigning a low probability to a random chance origin and a high chance probability to the God origin really just amounts to expressing your personal level of surprise about one and your comfort with the other. It has no objective bearing on the truth. As a previous poster put it, “if these are where these supposed "probabilities" are coming from, then it is equivalent to "some people subjectively suppose it's a very unlikely probability that the universe is as it is without our God to make it so."”
All of these probability claims seem impressive, but in the end, all they amount to is a person’s measure of their surprise that something would be true. So Collins’ claim, "On the God hypothesis, the fine tuning we observe in the universe is highly probable," really says little more than "I would find it very surprising that God doesn't exist in a world with these physical features."
The problem here is that one's subjective measure of surprise, to put it mildly, just doesn't count for jack. Medieval priests would have been exceedingly surprised to find out that the bubonic plague was caused by a bacteria, not by evil demon possession or the corruption of sin. On their view, this sentence seems justified: "On the sinners-are-punished hypothesis, the health problems we observe in plague victims would be very likely." Therefore, the plague is caused by sin. Copernicus’ contemporaries were exceedingly surprised and assigned a very low probability to his claim that the Earth orbits the sun. I know lots of people are very surprised to find out that the Gambler’s Fallacy is a fallacy. Many ancient people would assign a very low probability to the claim that the earth is spherical, not flat. And so on.
Since authors like Collins and Swinburne are using terms like "probability" lots of people are more impressed with the arguments than they should be. But what becomes clear when the details of Bayes Theorem come out is that the arguments are flagrantly circular. The existence of God is exceedingly probable because I find the non-God alternatives to be very improbable, therefore, the existence of God is exceedingly probable.