Fine tuning and probability

Luke Barnes criticizes this version of Ikeda and Jefferys' paper, "The Anthropic Principle Does Not Support Supernaturalism." (I usually link to this version; I assume the content is identical.) Barnes' post is five years old, but it's still be referenced by some people, so I guess it's still alive enough to be worth rebutting. In his criticism, Barnes runs afoul of some of probability's conceptual problems.

In general, probability is very philosophically problematic. Definitely with the frequentist interpretation, but also with the Bayesian interpretation, using probability requires us to adopt an ontological commitment to the existence of unknowable things: frequentism requires us to talk about experiments not performed, Bayesianism requires us to talk precisely about what we don't know. It is philosophically respectable to reject probability entirely: the past is fixed and certain, the future is determined by the present, and it is incoherent to measure our ignorance. However, if you're going to use probability to make inferences, you have to bite all the philosophical bullets that probability requires.

To briefly recap, the Fine Tuning Argument (FTA) says that, given relatively unproblematic assumptions about the laws of physics at the most general level, the probability is very low that the (apparently) arbitrary constants of the laws of physics, i.e. those constants whose values cannot be determined theoretically and must be determined observationally, are such that life can exist. We human beings apparently got ridiculously lucky when the universe formed almost perfectly set up for life to evolve. The Weak Anthropic Principle (WAP) rebuts the FTA: if the universe were not "fine tuned" for life, we would not be around to observe otherwise; of course we observe that the universe is "fine tuned." Barnes counter-argues that if we are required to condition on the result (life exists), then probabilistic arguments are generally useless. Barnes is correct: if we are required in principle to condition a probabilistic argument on the result, then probabilistic arguments are useless. However, he is not correct that Ikeda and Jeffries argue that we are required in principle to condition on the result.

We have to make additional assumptions to make probability work. First, we have to be conceptually able to "observe" both success or failures. If we cannot in principle observe failures, then we cannot make any inference at all about the probability of the successes we observe. This is a trick that "psychics" use: they show the hits but hide the misses. Second, we have to be conceptually able to describe success before we actually observe it. In a large probability space, every actual event has a very low probability. Shuffle a deck of cards and then look at it: before you shuffled it, the probability that the deck would end up in exactly that order is $1/{52!}≈1.24×10^{-68}$.

The difference between the FTA and Barnes' example is that in the FTA, we cannot in principle, even conceptually, observe a universe that does not have life in it. In contrast, I personally could in principle have observed Magneto and his grandson being killed by the shrapnel. Furthermore, even though the grandson could not have actually observed that outcome, he could conceptually have "observed" the alternative outcome, e.g. by observing that other people actually died.

Ikeda and Jeffries invert the FTA. Instead of asking the probability of naturalism, they ask the probability of God existing. We first assume first that if God does not exist, then life will exist only in a life-friendly universe. Life existing entails life friendliness $P(F|~G&L)=1$. We don't have to condition on L, but if we do, then if $G$ is false then $F$ must be true.

We then consider two alternative hypotheses. The first hypothesis is that if God₁ exists, then He would have created a life-friendly universe, If that is the hypothesized God, then observing life-friendly universe with life in it (our only actual observation) does not distinguish between the existence and non-existence of God. The second hypothesis is that if God₂ exists, then He would have created life, and might have created life in a non-life-friendly universe. In this case, Ikeda and Jeffries correctly argue that observing life in a life-friendly universe lowers (slightly) the probability that God₂ exists (in the same sense that observing a non-black non-crow raises the probability slightly that all crows are black).

We don't have to condition on life existing. However, we're worse off. We could not conceptually have predicted before observing any universe that life would exist in it. The only reason we have to assume that if God exists, He would have created life is just that we are in fact alive. We are "predicting" the random order of the deck after we've seen it. (Positing indirect attributes of God that entail His creating life just move the problem around.) Just the hypothesis that if God exists, he would have created anything we actually see begs the question: it assumes what we are setting out to "prove." Plantinga would call that an adequate argument, but Plantinga is a doofus who doesn't understand modal logic or probability.

In short, the Fine Tuning Argument for the existence of God is dead. It either provides evidence against the existence of God, or it fails to satisfy the requirements to even make a probabilistic argument in the first place.