Some physicists seem to think that the only good reply to the fine-tuning argument for God is an appeal to many universes. If that’s right, that puts the fine-tuning argument on pretty strong ground. Leonard Susskind is a physicist who falls into this camp. He says:

If, for some unforseen reason, the landscape [i.e., the many-universes version of string theory] turns out to be inconsistent — maybe for mathematical reasons, or because it disagrees with observation — I am pretty sure that physicists will go on searching for natural explanations of the world. But I have to say that if that happens, as things stand now we will be in a very awkward position. Without any explanation of nature’s fine-tunings we will be hard pressed to answer the ID critics. 

In fact, I think that the many-universes reply to the fine-tuning argument isn’t as strong as many people think, but there are other stronger ways of responding to the fine-tuning argument that don’t depend on there being many universes. For my defense of this, see my paper “God, Fine-Tuning, and the Problem of Old Evidence”. I wonder if Susskind has actually gone through the various other responses one could give to the fine-tuning argument, and concluded that they all are bad, and hence the many-universes reply is the only good one, or if Susskind has simply latched on to the many-universes reply, and hasn’t really thought about other responses.


I received an email from someone wondering whether I knew about the Ikeda/Jeffreys paper on the fine-tuning argument, since I didn’t talk about it in my paper “God, Fine-Tuning, and the Problem of Old Evidence.” Here’s how I replied:

Yes, I am familiar with the Ikeda/Jeffreys paper. I’m not completely sure I get their argument, because to be honest the paper is a mess structurally (at least, the online paper is, and it doesn’t appear that they’ve published a cleaned-up version). But if I am understanding their argument right, then I would say that they are making the same mistake Sober makes, and they’re not taking into account that we sometimes legitimately change our probabilitiy assignments in light of old evidence. In the fine-tuning case, the fact that the universe exists with life is old evidence, but nevertheless proponents of the argument want us to change our probability assignments in part as as a result of that old evidence. Ikeda and Jeffreys don’t want to allow that:

Please remember that if You are a sentient observer, You must already know that L is true, even before You learn anything about F or P(F|N). Thus it is legitimate, appropriate, and indeed required, for You to elicit Your prior on N versus ~N conditioned on L and use that as Your starting point.

But I don’t see their argument for why one can’t treat L as old evidence, and reason probabilistically using L (and one of the standard solutions to the problem of old evidence, such as the ur-probability solution I give in my paper “God, Fine-Tuning, and the Problem of Old Evidence”). I can think of various prima facie plausible arguments that could be given for why one can’t treat L as old evidence, but I haven’t seen Ikeda/Jeffreys or Sober give even a prima facie plausible one. The argument that Sober does give forces him to say that in the firing squad example, where the person about to be shot at survives the shooting unharmed, that we observers can reason that his survival probably happened as a result of design, but he can’t. This is (in my opinion, at least) a clearly unacceptable result. That’s why I criticize it in my “God, Fine-Tuning, and the Problem of Old Evidence” paper, and Jonathan Weisberg also gives a good criticism of it in his BJPS paper “Firing Squads and Fine-Tuning: Sober on the Design Argument”.

On a different topic, I pretty much agree with the Ikeda/Jeffreys claim that an omnipotent creator could cause us to exist in any universe whatsoever, if the idea is that, even if the physical constants don’t have the right values to lead to a universe that’s friendly to standard embodied life, nevertheless God could put life in there, and ensure through occasionalist-style intervention that that life survives. This raises the point that there are two different theistic hypotheses one could consider — the hypothesis that God exists and God wants life, and the narrower hypothesis that God exists and God wants life existing in a universe in the standard sort of way that we’re familiar with. The fine-tuning proponent would say that the evidence we have supports both hypotheses (and presumably that it supports the second hypothesis more), but it could well be the case that the prior probability for the second hypothesis is lower than the prior probability for the first.