The Argument from Design Meets a Third Contender, and Bayes

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by Neil Godfrey

William Paley
William Paley

In crossing a heath, suppose I pitched my foot against a stone and were asked how the stone came to be there, I might possibly answer that for anything I knew to the contrary it had lain there forever; nor would it, perhaps, be very easy to show the absurdity of this answer.

But suppose I found a watch upon the ground, and it should be inquired how the watch happened to be in that place, I should hardly think of the answer which I had given, that for anything I knew the watch might have always been there.

Yet why should not this answer serve for the watch as well as for the stone; why is it not admissible in that second case as in the first?

For this reason, and for no other, namely, that when we come to inspect the watch, we perceive — what we could not discover in the stone — that its several parts are framed and put together for a purpose, e.g., that they are so formed and adjusted as to produce motion, and that motion so regulated as to point out the hour of the day; that if the different parts had been differently shaped from what they are, or placed in any other manner or in any other order than that in which they are placed, either no motion at all would have carried on in the machine, or none which would have answered the use that is now served by it. (William Paley, Natural Theology, p. 1)

William Paley’s famous argument for creation by a designer consists of two distinct arguments joined together:

  • Artefacts like watches and living organisms like eyes have special functions. Watches to tell the time; various kinds of eyes to see in various types of environments: “each such entity exists because of its function” (p. 42);
  • Such functionality implies a designer both conscious and intelligent.

Biologists accept the first argument.

The second proposition seems right given the axiom that a cause must precede every effect. The effect is the ability to see. It must therefore follow that the eye was caused to exist for this specific function. In other words we have a teleological argument for the existence of eyes. They appeared for the purpose of enabling sight.

According to Paley there are only two alternatives. A complex organism, such the eye, must have come about either by

1. a conscious designer


2. blind chance aided by no other mechanism

Sahotra Sarkar
Sahotra Sarkar

Sahotra Sarkar (whose argument this post follows) comments in Doubting Darwin:

If these are the only two possible hypotheses we may still prefer the second because we have never had independent experience of a designer of eyes. But, at least, we would have to concede that the first is not unreasonable. (p. 44)

Enter a third contender

Introduce evolution by the process of natural selection, however, and these are no longer the only two alternatives.

Natural selection changes everything. The “every effect must have a cause” argument is no longer so clear-cut. An organ sensitive to light may be a blind chance mutation but the fact that it survives and is passed on will be the consequence of its sensitivity to light enhancing its ability (or the ability of the larger organism it is part of) to survive.

In this case the ability to see among so many creatures is not because they have eyes that were created in order to see (or that evolved in order to see). Rather, so many creatures today have eyes because random mutations that produced sensitivity to light happened to increase their chances of survival.

The eye is here today because it was selected by natural processes as the better survivor. In another environment in another time it may be replaced by something else that bestows better survival skills.

Natural selection breaks down the teleological argument for the existence of eyes. Eyes did not emerge for the purpose of seeing; they are “not fearfully and wonderfully made” by God. Their ubiquity is the natural result of how they functioned in the blind circumstances of the environment.

crystals-stalactPaley could see only two options: a designer or blind chance. However,

Chance may be aided by a potentially infinite number of natural processes. 

Consider complexity of design. It is easy to assume that this must exist as a result of some intelligently planned purpose — until we recall crystals. Complexity in the design of crystals is not there to serve any particular purpose or function.

The apparently goal-driven activity of some natural processes may in fact be merely an illusion. Stalactites and stalagmites grow towards each other as if the two are seeking to meet yet this appearance of goal-directed action can be explained more simply as the natural process of other forces at work.

When we introduce natural selection into the question posed by Paley we no longer have two alternatives but three.

Natural selection explains the functions of organisms without any need to think that they emerged for “the purpose” of carrying out those functions.

Three hypotheses are on the table now:

1. conscious design

2. blind chance alone

3. blind chance coupled with natural selection

Now we see that the blind chance alone option is not going to attract many bets. It is clearly less likely than the other two contenders.

Enter Bayes

Sahotra Sarkar compares the probabilities of conscious design and blind chance with natural selection being responsible for a complex organ like the eye. He applies Bayesian reasoning noting that this also appeared to be the approach Paley himself usually had in mind.

As we get more knowledge of the world, we update these probabilities in the light of new data. This is what philosophers call a Bayesian perspective. . . . (p. 45)

The first step in Bayesian method is to take stock of all our relevant background knowledge. What do we already know about each of our options?

090212_darwin_galapagos_msAs for natural selection

we have some evidence about what [it] is and what it may produce.

No-one, not even Creationists as far as I am aware, dispute the evidence for adaptation found on the Galapagos Islands.

And for conscious design

we have no evidence about the existence or abilities of potential designers of adaptations. (We cannot point to the adaptations themselves: we need independent evidence about what these designers can and cannot do — otherwise our reasoning becomes circular.) (my bolding)

Because we have some evidence that natural selection can produce other adaptations and none for the designer we should assign a higher initial probability to the natural selection option. Sahotra, generous soul that he is, bends the rules in favour of the Creationists: he assigns equal prior probability to both.

Then we must show that the probability of the hypothesis of a designer, given our observed adaptation, is greater than that for the hypothesis of natural selection, given the same observation. (p. 46)

What the Creationist needs to establish, then, is that given the hypothesis of a designer the probability of observing this adaptation is greater than observing the same adaptation given the hypothesis of natural selection.

But this is precisely what we cannot establish. The theological purpose of the design argument is to establish the existence of a designer sufficiently powerful and different from us to be identified with some god. Even if we tame most of our theological aspirations, at the very least, we must show that the putative designer must be a designer of natural adaptations, and not only clocks. But, as Sober and others have correctly pointed out, we do not have independent access to what the goals and abilities of such a designer are and, once again, we need independent access to these features before we can begin assessing the relevant probabilities.

We do not know if the adaptation could or should have been better – or, perhaps, worse, given the ultimate goals of the designer. We have no way to use our observation of an adaptation to estimate probabilities about what such a designer may or may not accomplish.

We may or may not want to believe in the existence of such a designer but the observation of adaptations is irrelevant to the question. 

As Darwin asked: “Have we any right to assume that the Creator works by intellectual powers like those of man?”

(In passing, we should also notice that Paley’s assumption that the stone was not put in the place where he found it by the divine designer for some cosmic purpose is also unwarranted. We do not even know that – Paley’s argument may be even worse than how it is usually presented, even by its critics.) (p. 46, my formatting)


Incoherence and “the only rational theological claim”

Biologists accept the argument for functionality for adaptations.

But any argument from functionality to a conscious designer is not only unsound, but because we cannot assign the relevant probabilities, simply incoherent. Meanwhile natural selection acting on blind variation gives us a handle on the origin of organic adaptations. (p. 48, my bolding)

John Henry Newman
John Henry Newman

Sahotra Sarkar concludes with what he describes as “perhaps the only rational theological claim that can be made about the design argument”:

I believe in design because I believe in God; not in a God because I believe I see design. (John Henry Newman, Letters and Diaries, Vol XXV, p. 97)

The argument applies just as strongly to believers — Christians, Jews, Muslims — who profess to believe in evolution while simultaneously believing that God somehow planned a destiny for or a relationship with humans. This belief is a more subtle form of Creationism that fails to understand evolution by means of natural selection.

Sarkar’s presentation of the argument for evolution by natural selection as well as the number of potential options he concedes as possible alternatives to the design hypothesis are slightly more nuanced than the ones I have presented here. But I don’t think I have done any injustice to the main argument.



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Neil Godfrey

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4 thoughts on “The Argument from Design Meets a Third Contender, and Bayes”

    1. Great. Do you think we should ask Professor McGrath if he would like to link to your page and my post given his strong interest in combatting Creationism. They might even expand his imagination to see that one doesn’t have to keep laughing at Creationists and telling them they are stupid in order to argue the case.

  1. Change the word ‘watch’ in Paley’s sentence to an equally appropriate descriptive phrase that conveys the same meaning and intent for the writer and
    things change a tad.
    Eg ‘But suppose I found a ‘device for telling the time ‘upon the ground ….”
    Not necessarily one of these:
    but maybe one of these:
    Or even, in its most primitive form .. an upright near vertical stick in the ground casting a shadow. Or perhaps a boulder, a large stone, whose shadow provides cool in the morning heat on one side and afternoon cool on the other side.
    Hmm small increments of improvement of this phenomena may be possible, over time by multiple designers.
    As this article describes:

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