The Sydney Morning Herald is reporting on a lecture given by an Oxford academic to the Australian Catholic University in which it was said that “It is 97 per cent certain God raised Jesus from the dead – based on logic and mathematics, not faith”. According to the story:

Professor Swinburne, who gave a public lecture at the Australian Catholic University last night, said probability calculus showed a probability of 97 per cent. The probability God existed was one in two. That is, God either did or didn’t. And it was one in two that God became incarnate.

Professor Swinburne suggested a one-in-10 probability that the gospels would report the life and resurrection of Jesus as they did. The chance of all these factors coming together, if the resurrection was not true, was one in 1000.

Now, assuming the good Professor was accurately quoted, this is pretty disappointing stuff, because it mis-states possibly the most fundamental part of probability theory: confusing the number of options that could potentially happen versus the probability that any of those options will actually happen.

It’s correct to say that there are two options, that God existed, and that God didn’t exist. But it’s totally wrong to say that, therefore, the probability of each option is 50%.

By the same reasoning:

  1. every horse in a horse race will either win, or not win. On the quoted logic the chance that each horse will win is 50%.
  2. I will either win the gold medal for the 100m sprint at the next Olympics, or I won’t. So the chance I will win that gold medal in Beijing in 2008 is 50% (start betting on me! I will offer good odds!)
  3. The odds would also be 50:50 that I would win the women’s 100m sprint at the next Olympics — even if, according to the rules, I would be ineligible for the event.

I then leave aside the other problem, of assigning fanciful probabilities to things that can’t be quantified — namely, the chance that the gospels would report what they do “as they did”, etc. For the sake of (one of) my alma maters, I hope he was joking, or his speech was misrepresented.