#-991 Formulating the Argument from Fine-Tuning

Dr. Craig, I have been thinking recently about the fine-tuning argument, especially in Bayesian terms. Normally, even if one begins with extremely skeptical priors for theism (for example, one in an octillion), the probability of the evidence on theism appears so much greater than on naturalism that the posterior probability of theism becomes quite high once the fine-tuning of the universe is taken into account.

However, skeptics such as Alex O'Connor have recently raised a more sophisticated critique. According to this objection, if God is metaphysically capable of actualizing any feasible world, then it seems entirely possible that He could have created a universe in which the numerical values of the fundamental constants—such as the gravitational constant or the electromagnetic coupling—were very different from those in our universe, while the world still possessed life and redemptive value. In other words, there may be an enormous (perhaps effectively infinite) range of possible universes with quantitatively different physical parameters that would still allow God to realize His purposes in creating embodied rational agents. The specific physical modality, qua modality, would be different, but the goods being actualized could (it seems) be virtually indistinguishable from those realized in the actual world.

The skeptic’s question, then, is this: why should theism predict this particular physical modality—with these precise values of the constants—rather than any of those other life-permitting possibilities? If theism is compatible with a vast range of different life-permitting physical structures, it might seem that the probability of observing these exact values given theism is extremely small as well. After all, we are in no position to read the intentions of an infinite mind whose range of creative possibilities may be immense. In that case, the evidential advantage that fine-tuning is supposed to give theism in a Bayesian comparison may appear to diminish.

A simple analogy may help illustrate the concern. Suppose an architect intends to build a house suitable for human habitation. There are countless possible blueprints that would produce a livable house. If we discover that the architect chose one extremely specific blueprint among millions of equally viable designs (representing different physical modalities where the numbers, though different, still produce intelligent life), the mere fact that the house is livable might not strongly confirm that particular blueprint, since many other designs would have achieved the same goal. Similarly, if God could have realized life through countless different sets of physical constants, why should theism strongly predict this specific configuration rather than any other life-permitting one?

How might a theist address this difficulty within a Bayesian framework? More specifically, should the relevant likelihood be understood as the probability that the universe is life-permitting given theism, or as the probability that it would possess these exact values of the constants? If theism merely predicts that some life-permitting universe will exist, but is largely indifferent among the many possible sets of constants that could realize that condition, then it might seem that the probability of observing this particular configuration given theism is quite small. Would it therefore be legitimate to treat the physical modality we inhabit—together with its laws and constants—as a fixed background condition when evaluating the likelihoods, so that Bayes’ theorem still yields the traditional fine-tuning result? Or would that move risk appearing ad hoc?

Thank you,

Sam