I’ve been looking at Alvin Plantinga’s book Where the Conflict Really Lies: Science, Religion, and Naturalism and I appreciate his modest assessment of the fine tuning argument (FTA). It’s refreshing to see a Christian philosopher give a sober assessment of this argument instead of jumping on the apologist’s bandwagon. I’ve come to think that the FTA depends upon the ‘wow’ factor of dizzying numbers and stupefying ‘improbabilities.’ Human beings, however, are notoriously bad at making off-the-cuff probability judgments. When one tries to rigorously determine the probability of fine tuning, it becomes a lot less clear cut. Disclaimer: I’m no expert in probability theory or Bayes’ Theorem. I do, however, try to consult the relevant experts when forming my opinion about such matters.
With that in mind, I want to take a look at some puzzling features of the FTA when one tries to formally state the relevant probabilities. For the purposes of this post I will disregard the popular multiverse objection to the FTA. The probabilities I discuss all assume a single universe. First, there is the question of whether the probability space is normalizable. I’ve dealt with this question before, so I won’t belabor the point now. Briefly, the idea behind this objection is that in order to meaningfully assign probability, the ‘probability space’ must add up to 1. For example, the probability of rolling a number between 1 and 6 on a six-sided die is 1 (1/6 x 6 = 1). But in the case of the fundamental constants of physics, there doesn’t seem to be a way to make the relevant probabilities add up to 1 because the probability space is infinite. There appear to be no logical limits on the possible values of these constants.
But let’s assume that this objection can be overcome. Second, there is the Bayesian approach to the FTA. Plantinga sets it up as follows: P(T/F) = P(T) x P(F/T)/P(F) where T is theism and F is the proposition that the universe is fine tuned. P(T) and P(F) are the antecedent probabilities of theism and fine tuning. This is an important point. The antecedent probability of theism is the probability of theism prior to the evidence of fine tuning. The antecedent probability of fine tuning is the probability of fine tuning prior to the discovery of evidence that the universe is in fact finely tuned. But how does one estimate these antecedent probabilities? There seems to be no objective answer to this question.
This problem leads Elliot Sober (atheist) and Robin Collins (theist) to propose a likelihood version of the FTA. In this version, no reference is made to antecedent probabilities. Rather, we simply compare P(F/T) with P(F/A) where A is atheism. As Plantinga puts it: “If fine tuning is more to be expected given theism than given atheism, then the existence of fine tuning confirms theism over atheism” (WTCRL, 220). But how does the theist know what to expect of an omnipotent God? Is it not possible that God would create a universe with a very wide range of life permitting variables so as to make the existence of intelligent life a near certainty? The factual premise of the FTA — that the variables are balanced on a razor’s edge — seems to severely limit God’s creative freedom. Given fine tuning, if God wanted intelligent life, he had no choice but to create the universe the way he did. In this sense, the probability of the universe being fine tuned is 1. Perhaps we’re smuggling in our background knowledge here, although I don’t see any way to avoid doing so. But if we do this with P(F/A) we get the same result because part of our background knowledge is the fact that there is intelligent life in the universe. Therefore, the probability that the universe would be life-permitting is 1. Thus, the probability is 1 on either theism or atheism.
Granted, the likelihood version of the FTA would have us ignore our background knowledge along with antecedent probabilities. But it seems to me that sauce for the goose is sauce for the gander in this case. The FTA proponent needs a stronger construal of T than ‘there is such a person as God’ in order to base any expectations on it. As Elliot Sober says, “The problem is that the design hypothesis confers probability on the observation only when it is supplemented with further assumptions about what the Designer’s goals and abilities would be if He existed” (Quoted in WTCRL, 220 — 21). So the theist has to add these further assumptions to the concept of God and, presumably, these assumptions form part of the theist’s background knowledge. Therefore, the theist needs something at least as strong as what Plantinga calls T* ‘There is such a person as God and he wants there to be life.’ (I’d argue that the theist needs something much stronger to justify fine tuning over, say, ‘coarse tuning’ but be that as it may.) But, as Plantinga also notes, the atheist can come back with A* ‘There is no such person as God and the universe has a powerful intrinsic impulse toward the existence of life.’ This can go on and on, but because we don’t have antecedent probabilities to constrain these hypotheses, this one-upmanship is interminable.
Plantinga concludes that the likelihood version of the FTA ‘is pretty anemic’ (WTCRL, 222) but says the same for the Bayesian version. Even if we consider the antecedent probabilities, due to the modal aspects of theism, the theist is going to say that the probability of theism is 1 and 1 on any evidence. Likewise, the atheist will say that the probability of theism is 0 and 0 on any evidence. If we switch from objective probability to epistemic probability (how much credence we should place in a belief) problems remain. The theist and the atheist are going to assign different subjective probabilities to the priors. In other words, we haven’t made any progress.
So what’s the value of the FTA? I think it certainly has rhetorical value in the apologist’s arsenal. That’s why we are unlikely to see it disappear any time soon. Again, the dizzying numbers and stupefying ‘improbability’ is difficult for the layman (theist or atheist) to resist. In Plantinga’s more modest assessment, the FTA provides some slight support for theism depending on how one construes the epistemic probabilities. In the end, then, the FTA is like most philosophical arguments: its force is subjective. It will move some, but not others. As Plantinga says elsewhere, ‘that’s just life in philosophy.’