Is Fine-Tuning Evidentially Irrelevant? Weisberg Says "Yep"

Jonathan Weisberg argues that the discovery of a finely-tuned universe provides no support for Theism [1]. Here's why. We now know that there are vastly more ways our universe could have prohibited life than permit it. Call this New Datum. That is, the range of life-permitting values for the various physical constants in the universe (e.g., the gravitational constant, the expansion rate, etc.) is really narrow compared to the range of life-prohibiting values these constants could have taken. But before we knew this, Weisberg notes, we already had reason to believe that our universe was life-permitting. After all, we knew that we existed. And given that we have embodied existence, we must live in some life-permitting universe or other. So our background knowledge of the world already included the fact that our universe is life permitting. Call this piece of data Old Datum. 

However, Old Datum doesn't predict whether our universe will be "barely" life-permitting or not. Some life-permitting universes are "barely" life-permitting. Of all the values that the physical constants in those universes could take, only a very small fraction are life-friendly. On the other hand, some life-permitting universes are "widely" life-permitting. The range of life-friendly values for the constants in those universes is really large. Does learning that our universe is barely life-permitting give us some evidence for design? To answer this question, ask yourself this: If God were going to create a life-friendly universe, how likely is it that she would make it a barely-life-permitting universe rather than a widely-life-permitting universe? And if a life-permitting universe were to come about by chance, how likely is it that it would be a barely-life-permitting universe rather than a widely-life-permitting universe. According to Weisberg: 

Given only that her aim is to create intelligent life, is one of these options particularly preferable? It seems not. So there is no reason to expect her to choose a finely-tuned cosmology over one of the alternatives. Once we know that intelligent life exists, we know that some life-allowing cosmology must obtain, and a fine-tuned cosmology seems no more likely given a designer than given not.

As a result, learning that we live in finely-tuned universe -- i.e., a barely-life-friendly universe -- offers no support to Theism. That's the argument.

Here's the formal side of the argument. Let B = "our universe is barely-life-permitting" (i.e., New Datum), let D = theism/design hypothesis. In claiming that fine-tuning is evidence for D, the argument can be understood as saying the following:

Pr(B|D) > Pr(B|~D)

A barely-life-permitting universe is more likely given design than not. And if that's the case, B is evidence for D. But Weisberg draws our attention to the presence of Old Datum in our background knowledge. The inequality should look like this:

Pr(B|D & Old Datum) > Pr(B|~D & Old Datum)

But this inequality is false, for the reasons given above. We have no reason for thinking that God would choose B over ~B or that chance would "choose" B over ~B. I suppose Weisberg thinks we should apply the principle of indifference to the available options, though he doesn't state this explicitly in the paper. The principle of indifference says that if you have no reason for thinking that any one outcome in a class of possible outcomes is more likely to occur than the others, you should assign equal probabilities to each. Given that there are only two possible ways a universe could be life-permitting -- barely vs widely -- and absent a reason to think God or chance would prefer one over the other, we should assign B and ~B equal probabilities on each hypothesis, D and ~D. As a result:

Pr(B|D & Old Datum) = Pr(B|~D & Old Datum) 

And hence, learning that we live in a barely-life-permitting universe offers no support for D. 

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Footnote:

[1] All quotes and references are from Weisberg, "What's Fine-Tuning Got to Do With It?" (2010).