Plantinga on Fishnets and Fine-Tuning

According to perhaps the strongest design argument for God’s existence, the “fine-tuning” argument (FT, hereafter), we start with the observation that life-supporting constants fall within a very narrow range, and then infer that it does not seem likely that they would turn out this way by chance. For example the ratio of protons to electrons, both of which were formed at different times, could not have deviated from one part in 1037. Another is in the rate of cosmic expansion, which could not differ from what it is now by one part in 1060. These numbers are meant to suggest that it is more likely that God designed the universe to come about the way it did, than it would have if it came about by chance.

Elliot Sober (following Arthur Eddington) objects to this sort of argument. How does he go about it? Well, he begins where the design argument ends with what he calls a “likelihood inequality,” and styles it the following probability formula:

  1. Pr(constants are right │ Design) > Pr(constants are right │ Chance).

This just means that the probability of the constants being right because of design is greater than the constants being right because of chance. However, he thinks this is analogous to an argument concerning fish being caught in fishnets.

  1. Pr(All the fish I caught were more than 10 inches long │ All the fish in the lake are more than 10 inches long) > Pr(All the fish I caught were more than 10 inches long │ Only half the fish in the lake are more than 10 inches long).

So far so good, but what if we know the following background assumption?

  • A1: The fish were caught with a net that can’t catch fish smaller than 10 inches.

This counts as an “observational selection effect” (OSE, hereafter), which is just a fancy way of saying ‘selection bias.’ The problem is that the procedure for gathering evidence skews our observation so that we get a predetermined result. Thus, when we take this into account,

  1. Pr(All the fish I caught were more than 10 inches long │ All the fish in the lake are more than 10 inches long & A1) = Pr(All the fish I caught were more than 10 inches long │ Only half the fish in the lake are more than 10 inches & A1) = 1.0.

Now we are in a position to see the heart of Sober’s analogy. He considers A1 to be analogous to the weak anthropic principle:

  • WAP: We exist, and if we exist the constants must be right.

As observers we can’t help but evaluate competing theories with the assumption that conditions for our existence as observers have been met. Because we cannot observe a universe that failed to meet the conditions necessary for our existence, we should not take those possibilities into account when assessing the evidence. Hence,

  1. Pr(constants are right │ Design & WAP) = Pr(constants are right │ Chance & WAP) = 1.0

Therefore, we shouldn’t be surprised that the constants are just right for life, because if they weren’t we wouldn’t be here to be surprised!
Alvin Plantinga, in his new book Where the Conflict Really Lies, has a great response to this sort of maneuver. He writes:


Let’s suppose there are (or could be) many universes (see pp. 210ff); use “alpha” as a name of the universe we find ourselves in, the universe that is comprised by everything that is spatiotemporally related to us. What we observe is:
  • O: Alpha is fine-tuned.

We have the following two hypotheses:

  • D: Alpha has been designed by some powerful and intelligent being,


  • C: Alpha has come to be by way of some chance process that does not involve an intelligent designer.

We note that O is more likely on D than on C; we then conclude that with respect to this evidence, D is to be preferred to C. Granted: we could not have existed if alpha had not been fine-tuned; hence we could not have observed that alpha is not fine-tuned; but how is that so much as relevant? The problem with the fishing argument is that I am arguing for a particular proportion of ten-inch fish by examining my sample, which, given my means of choosing it, is bound to contain only members that support the hypothesis in question. But in the fine-tuning case, I am certainly not trying to arrive at an estimate of the proportion of fine-tuned universes among universes generally. If I were, my procedure would certainly be fallacious; but that’s not at all what I am doing. Instead, I am getting some information about alpha (nevermind that I couldn’t have got information about any other universe, if there are other universes); and then I reason about alpha, concluding that D is to be preferred to C. There seems to be no problem there. Return to Eddington’s fishing example, and suppose my net is bound to capture exactly one fish, one that is ten inches long. I then compare two hypotheses:

  • H1 this fish had parents that were about 10 inches long


  • H2 this fish had parents that were about 1 inch long.

My observing that the fish is ten inches long is much more probable on H1 than on H2; H1 is therefore to be preferred to H2 (with respect to this observation). This argument seems perfectly proper; the fact that I couldn’t have caught a fish of a different size seems wholly irrelevant. The same goes for the fine-tuning argument.


From: Plantinga, Alvin (2011-10-26). Where the Conflict Really Lies:Science, Religion, and Naturalism (pp. 203-205). Oxford University Press. Kindle Edition.

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