This is the weekly Q & A blog post by our Research Professor in Philosophy, Dr. William Lane Craig.


Recently, I have run across arguments by atheists that say that the universe is not fine-tuned for life, but life is fine tuned to the universe and they describe this in the "puddle" analogy (which I'm sure you are aware of). I searched your website for something addressing this argument, but maybe did a 'bad job' searching, Could you give your rebuttal to this or point me to a place in your website that you speak to this subject; as this argument has been bothering me and I don't know how to address it. Thank you very much for your work you do and for your time.


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Dr. William Lane Craig's Response

Dr. William Lane Craig

As stated, the objection embodies a number of confusions. First, in asserting that “the universe is not fine-tuned for life,” the objector seems to misunderstand the term “fine-tuning”. This is a technical term for the fundamental constants and quantities of nature’s falling into a very narrow life-permitting range. But the objector seems to take the term to mean “designed”. He wants to say that just as the hole in which a puddle exists is not designed for the puddle, so the universe is not designed for us and is therefore not fine-tuned. This is just wrong-headed. The term “fine-tuning” is a neutral term that says nothing by way of explanation of the observed fine-tuning. Otherwise attempts to explain the fine-tuning of the universe by chance or necessity would be self-contradictory. The fact that the universe is fine-tuned for embodied, conscious agents is well-established in physics and so scarcely disputable. The question is how best to explain fine-tuning: necessity, chance, or design?

The further claim that “life is fine tuned to the universe” is unintelligible to me. The objector cannot mean “designed,” lest he subvert his objection. But then is he saying that in order for the universe to exist, life’s parameters must fall within a narrow universe-permitting range? That doesn’t even make sense. I suspect that the objector’s assertion is just a catchy but meaningless slogan.

The reason you couldn’t find anything on the puddle analogy is not because of “a 'bad job' searching,” but because you didn’t recognize this old objection in fancy new masquerade. The puddle analogy is just our old friend the Anthropic Principle again. One is appealing to a self-selection effect in order to eliminate surprise at what one observes, however improbable. The anthropomorphized puddle can be surprised at its own existence only if the hole exists. So it shouldn’t be surprised by how well the hole and it fit. If the hole were not there, the puddle would not exist to be surprised about it.

There are multiple failings of the analogy. For example, the analogy suggests that we are trying to explain why this universe (this puddle) exists.[1] But we’re not. We’re trying to explain why a life-permitting universe exists. The analogy would be asking why puddles exist. Puddles can be any shape or size, so that there is no fine-tuning for puddles to be explained. The analogy collapses.

Moreover, proponents of the Anthropic Principle recognize that the principle can be legitimately employed only in conjunction with a World Ensemble (or multiverse) Hypothesis. The multiverse may be tacitly presupposed by the objector, since there are obviously a multitude of puddles in existence, coming in different shapes and sizes. But now the objector has launched his barque into the metaphysical deep and will need to defend his World Ensemble Hypothesis against objections.

Finally, and most importantly, the supposed self-selection effect for embodied agents is vacuous. As Robin Collins has explained, there’s no good reason to think that only fine-tuned universes have observers in them. This is the point of the famous Boltzmann Brain problem. Worlds which consist of a single observer with an illusory perception of an external world about him are physically possible. So how can the objector prove that we are not in such a Boltzmann Brain world? Since he has no way of doing that, his claim that we, like the puddle, can observe only worlds fine-tuned for our existence fails and with it his objection to the argument for design.


[1] The objection also seems to presuppose that the shape and size of the puddle are essential to its existence, which seems false.

This Q&A and other resources are available on Dr. William Lane Craig's website.