03 April 2011

William Lane Craig and probability theorems

In several of his debates, including the most recent one with Laurence Krauss, Dr. Craig trotted out a familiar canard: the use of probability theory to support his belief that God's existence is more likely than not. A typical example, taken from an article over at his website:

For in contemporary cosmology the heated debate surrounding the fine–tuning of the universe and the so–called Anthropic Principle will be greatly clarified by Dembski's Law of Small Probability.
Consider the application of the above Generic Chance Elimination Argument to the fine–tuning of the universe:
  1. One learns that the physical constants and quantities given in the Big Bang possess certain values.
  2. Examining the circumstances under which the Big Bang occurred, one finds that there is no Theory of Everything which would render physically necessary the values of all the constants and quantities, so they must be attributed to sheer accident.
  3. One discovers that the values of the constants and quantities are incomprehensibly fine–tuned for the existence of intelligent, carbon–based life.
  4. The probability of each value and of all the values together occurring by chance is vanishingly small.
Wait! Stop right there. Let's have a little thought experiment.

Imagine I show you a deck of cards. I shuffle the deck, and then ask you, What is the probability that I will pull any given card – say, an ace of spades – from the deck? You would likely answer, correctly, that the probability is 1 in 52. But what if you were unaware that I was using a trick deck, and all of the cards were the ace of spades? Clearly, you'd be wrong in your probability assessment. The reason you know that the probability of a given card is 1 in 52 is because you already know that a standard deck of cards contains 52 different cards – in other words, you know the range. If I change the range by using a trick deck, you can no longer calculate the probability.

This carries important implications when it comes to cosmology. Craig is essentially asking what the probability is that the universe would come into being with the specific laws and physical constants that it contains, and concludes that, "The probability of each value and of all the values together occurring by chance is vanishingly small." But here's the elephant in the room: nobody knows the range. No one knows the range of possible physical constants, or possible physical laws and properties that could form universes. The range may be very small or infinitely vast. We. Don't. Know. Craig's probability argument is a classic case of garbage in, garbage out.


Craig also tries to use probability theorems to validate the resurrection. It's more garbage in, garbage out. In his debate with Bart Ehrman, Craig pulls this masterpiece of obfuscation out of his butt:
Pr (R/B&E) = Pr (R/B) × Pr (E/B&R)   /   Pr (R/B) × Pr (E/B&R) + Pr (not-R/B) × Pr (E/B& not-R)

Basically, Pr (not-R/B) × Pr (E/B& not-R) represent the intrinsic probability and explanatory power of all the naturalistic alternatives to Jesus' resurrection. So the probability of Jesus' resurrection relative to our background information and the specific evidence is equal to this complicated ratio.
Now, one might reasonably ask how in the hell Craig finds quantities for things like "background information". But it's okay – he doesn't! It's a meaningless bit of misdirection Craig uses to confound the sheep. Eventually he gets around to explaining his rationale:
Dr. Ehrman just assumes that the probability of the resurrection on our background knowledge [Pr(R/B)] is very low. But here, I think, he's confused. What, after all, is the resurrection hypothesis? It's the hypothesis that Jesus rose supernaturally from the dead. It is not the hypothesis that Jesus rose naturally from the dead. That Jesus rose naturally from the dead is fantastically improbable. But I see no reason whatsoever to think that it is improbable that God raised Jesus from the dead.
Wait a second. It's bad enough that Craig is trying to determine probabilities without quantifying any values, but now he's just saying that it's magic! How do we quantify the probability of supernatural events? We can't, of course. If God can do anything he wants, what's the point in talking about probability at all?

William Lane Craig is a blackbelt in bullshit. He's crafty because he has a knowledge of some esoteric subjects that exceeds that of most laypersons, so it's easy for him to confound skeptics with an onslaught of obfuscatory exposition. But the emperor has no clothes, and the way to prove it is not by getting bogged down in the minutiae, but by paying close attention to the core assumptions underlying his prose.

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