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. Fine-tuning and the prior probability of Theism

Fine-Tuning and the
Prior Probability of Theism


I'm taking a philosophy class at a local community collage. I wrote a paper on the fine-tuning argument presented by Robin Collins. The fine tuning is improbable given atheism. Here we have all of the number for fine tuning for life given by Collins. But my professor raised an objection that I have never heard before, namely what is the probability of God? His objection is that if have a probability for the fine tuning we need a probability to compare it to. Since we don't have a clear one, why should we conclude that God is more likely than atheistic fine tuning? If you could help me understand this I would greatly appreciate it. I can understand that it seems like a very reasonable thing to think God is not as unlikely as the fine tuning but is there a strong philosophical argument or case to be made here?

United States

Dr. Craig responds:

Your professor’s objection will be more comprehensible if we put it into the context of the probability calculus. Let’s compare the probability of theism vs. atheism relative to the fine-tuning of the universe. Let G stand for the hypothesis that God exists and ¬G for the hypothesis that God does not exist. Let FT stand for the fine-tuning of the universe. The comparative probabilities will be computed as follows:

Now what you argued in your paper was that the third ratio favored theism. The fine-tuning of the universe is considerably more probable given God’s existence than given God’s non-existence. The evidence is therefore strongly confirmatory of theism. So far so good!

What your professor wants to say is that your argument still does not prove that God's existence is more probable than His non-existence relative to the fine-tuning of the universe. That is the first ratio on the left-hand side of the equation. In order to show that this ratio favors theism, you also need to show that the prior probability of God’s existence is not greatly lower than the prior probability of God’s non-existence. That’s the middle ratio above. The idea here is that the greater explanatory power of a hypothesis can be offset by the enormous improbability of the hypothesis itself.

There are two responses you might make to this objection. First, you might deny that you were trying to prove that the first ratio favors theism. You might simply rest content with showing that the evidence we have is hugely confirmatory of theism. Collins, for example, points out that in science we often do not have any way of estimating the prior probability of a hypothesis, and so we simply rest with arguments showing a hypothesis’ greater explanatory power. It may be that the prior probability of the hypothesis is just inscrutable. You might even point out that in order to resist your confirmatory argument on behalf of theism, it’s the atheist who has the burden of proof of showing that theism is considerably more improbable than atheism.

Second, you could argue that theism is not intrinsically much more improbable than atheism. For the prior probabilities Pr (G) and Pr (¬G) are not, despite appearances, computed in a vacuum. Rather these probabilities are computed relative to our background information about the world. You subtract the information about the fine-tuning of the universe from what we know about the world, and whatever is left is our background information. That information will include the evidence featured in all the other theistic arguments, such as the contingency of the universe, the beginning of the universe, the objectivity of moral values and duties, the applicability of mathematics to the physical universe, intentionality, the facts concerning the resurrection of Jesus, and so forth. I think that we should be prepared to argue that the prior probability of theism is much greater than that of atheism relative to our background information. In that way we successfully argue that theism is much more probable than atheism given the fine-tuning of the universe.