Doctor Logic and Lydia McGrew on Likelilhoods, Design, and Probabilities

Doctor Logic: If an all powerful being were designing life, we don't expect descent, common descent, common composition or a gradual appearance of features and species. How many ways can a God create life in a universe? The number of ways a God can do this is vastly greater than the number of ways unguided evolution can do so. For example, gods don't even need to create life consistent with physical laws because they can create ghosts. There's no need for descent (birth) because God can make animals outright or create factories (no car has ever been born to another car). Even keeping the species the same and changing their natural histories and genomics gives a God vastly more options than evolution. I think theists would be tempted to say that there are infinitely more ways God could create life than ways that evolution could create life.


This is a simple problem in Bayesian reasoning. Finding ourselves in a world that is consistent with unguided evolution implies that the probability that we're designed is extremely close to zero.

In other words, if God exists, then there are a million ways in which God could create things, including Young Earth Creationism, etc. If atheism is true, then if intelligent life is going to emerge, it's got to emerge through naturalistic evolution. So, if the evidence is compatible with naturalistic evolution, then the evidence very strong supports naturalistic evolution, since this evidence is very likely given atheism and vanishingly unlikely given theism.

Lydia McGrew's paper on design and likelihoods might serve as a way for theists to respond here. Because God could do it a certain way doesn't mean that it would be reasonable for God to do so.

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McGrew on ECREE

From his essay on evidence.

Extraordinary Claims and Extraordinary Evidence


Another common slogan, also popularized by Sagan, is that Extraordinary claims require extraordinary evidence. Much depends, of course, on what counts as extraordinary, both in a claim and in evidence. It cannot be simply that a claim is unprecedented. At a certain level of detail, almost any claim is unprecedented; but this does not necessarily mean that it requires evidence out of the ordinary to establish it. Consider this claim: “Aunt Matilda won a game of Scrabble Thursday night with a score of 438 while sipping a cup of mint tea.” Each successive modifying phrase renders the claim less likely to have occurred before; yet there is nothing particularly unbelievable about the claim, and the evidence of a single credible eyewitness might well persuade us that it is true.
The case is more difficult with respect to types of events that are deemed to be improbable or rare in principle, such as miracles. It is generally agreed in such discussions that such events cannot be common and that it requires more evidence to render them credible than is required in ordinary cases. (Sherlock 1769) David Hume famously advanced the maxim that No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish (Beauchamp 2000, p. 87), which may have been the original inspiration for the slogan about extraordinary evidence. The proper interpretation of Hume’s maxim has been a source of some debate among Hume scholars, but one plausible formulation in probabilistic terms is that
P(MT) > P(~MT) only if P(M) > P(T~M),
where M is the proposition that a miracle has occurred and T is the proposition describing testimonial evidence that it has occurred. This conditional statement is not a consequence of Bayes’s Theorem, but the terms of the latter inequality are good approximations for the terms of the exact inequality.
P(M) P(TM) > P(~M) P(T~M) when both P(~M) and P(TM) are close to 1. There is, then, a plausible Bayesian rationale for Hume’s maxim so long as we understand it to be an approximation.

It does not follow that the maxim will do the work that Hume (arguably) and many of his followers (unquestionably) have hoped it would. Hume appears to have thought that his maxim would place certain antecedently very improbable events beyond the reach of evidence. But as John Earman has argued (Earman 2000), an event that is antecedently extremely improbable, and in this sense extraordinary, may be rendered probable under the right evidential circumstances, since it is possible in principle that

P(TM)/P(T~M) > P(~M)/P(M),

a condition sufficient to satisfy the rigorous condition underlying Hume’s maxim and the slogan about extraordinary events. The maxim is therefore less useful as a dialectical weapon than is often supposed. It may help to focus disagreements over extraordinary events, but it cannot resolve them.

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Probabilities, miracles, and design

I do believe that a case can be made for the claim that the Christian story about what happened in the life of Jesus makes more sense of the evidence than any possible naturalistic story, so long as you are willing to allow for a God who might do such a thing. 

But here Hume keeps coming back. If you base probabilities on what is most frequently found in nature, and you don't introduce the possibility of a non-human designer, then it looks as if the frequency of dead people who stay dead defeats anything but "extraordinary" (read virtually impossible) evidence for, say, a resurrection. 

But, the believer responds, causing the miracles in the life of Jesus seems like something a God might do. It makes sense from a theistic perspective, as opposed to, say, claiming that God caused a bunch of people to hallucinate, or caused a bunch of people to propagate a hoax that would ultimately result in them ending up on the kind of cross that Jesus was crucified on. 

But, the reply goes, likelihoods about what a divine agent might or might not do can't be brought in. They aren't based on experience, the way, say, the frequency of dead people who stay dead does. If you bring God in, you play a wild card. Anything goes. 

But, the theist replies, we can draw inferences about possible divine designers from analogy to human designers. 

That's why I think Lydia McGrew's paper on design and probabilities is relevant to this whole debate, which I linked to a few days ago.

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Lydia McGrew on identifying intelligent agents

This looks like an intriguing paper.

Abstract: It is often assumed by friends and foes alike of intelligent design that a likelihood approach to design inferences will require evidence regarding the specific motives and abilities of any hypothetical designer. Elliott Sober, like Venn before him, indicates that this information is unavailable when the designer is not human (or at least finite) and concludes that there is no good argument for design in biology. I argue that a knowledge of motives and abilities is not always necessary for obtaining a likelihood on design. In many cases, including the case of irreducibly complex objects, frequencies from known agents can supply the likelihood. I argue against the claim that data gathered from humans is inapplicable to non-human agents. Finally, I point out that a broadly Bayesian approach to design inferences, such as that advocated by Sober, is actually advantagous to design advocates in that it frees them from the Popperian requirement that they construct an overarching science which makes high-likelihood predictions.

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