Once more unto the breach, dear friends. (Another long fine-tuning post, I’m afraid …)

An oft-cited article on the fine-tuning of the universe for intelligent life was written by Michael Ikeda and Bill Jefferys, and goes by the title: “”The Anthropic Principle Does Not Support Supernaturalism”. It appears online here, and ~~to the best of my knowledge has not been published anywhere~~ has been published in “The Improbability of God“, edited by Michael Martin and Ricki Monnier (edit: 3/11/2010).

**IJ’s Argument**

Unless otherwise noted, quotes are from Ikeda and Jefferys (hereafter ** IJ**). Their central argument is as follows. Let:

L = The universe exists and contains Life.

F = The conditions in the universe are ‘life-Friendly,’ that is, the conditions in our universe permit or are compatible with life existing naturalistically.

N = “The universe is governed solely by Naturalistic law.” The negation, ~N, is that it is not governed solely by naturalistic law, that is, some non-naturalistic (supernaturalistic) principle or entity is involved. N and ~N are not assumptions; they are hypotheses to be tested.

L is, of course, true of our universe. For the sake of argument, IJ assume that F is true. N and ~N are taken to have an a priori non-zero probability of being true. Now, the anthropic principle roughly states that living observers must observe conditions that permit the existence of observers. IJ formulate this as:

. (1)

N appears in the expression just in case a supernatural agent decides to miraculously sustain life in a non-life-friendly universe.

Now, after dealing with the fallacious^{1} argument , IJ reach their Bayesian climax:

(Bayes Theorem)

(using 1)

(since )

Thus, the fine-tuning of the universe for intelligent life is at best irrelevant to the truth of naturalism, and could actually make it more likely. The fine-tuning of the universe, even if it is true, cannot support supernaturalism. Notice that all probabilities are conditioned on L. As IJ say:

… for an inference to be valid, it is necessary to take into account all known information that may be relevant to the conclusion. In the present case, we happen to know that life exists in our universe (i.e., that L is true). Therefore, it is invalid to make inferences about N if we fail to take into account the fact that L, as well as F, are already known to be true. It follows that any inferences about N must be conditioned upon both F and L … In inferring the probability that N is true, it is entirely irrelevant whether P(F|N) is large or small. It is entirely irrelevant whether the universe is “fine-tuned” or not. Only probabilities conditioned upon L are relevant to our inquiry.

I have two responses. Here I will contend that IJ’s formulation of the argument is incomplete. In the second part, I’ll raise a few issues with this “conditioning on everything” idea.