Probability can’t explain it
Some recent email correspondence prompts me to revisit an important issue. Those who deny determinism often cite radioactive decay as proof of indeterminism. They say that an atom that actually decayed at time t might not have decayed at t given identical prior conditions. I’ve said that this view implies that there was literally no reason the atom decayed at t rather than not.
A correspondent objected that there is a reason: The laws of physics assigned a higher probability to decay than to non-decay. While not guaranteeing decay, the laws made decay more likely than not.
But if, as indeterminists say, the atom might not have decayed given the prior conditions, then the reason it decayed rather than not can’t be that decay was antecedently more probable. That’s because the antecedent probability would still have been what it was if the atom hadn’t decayed. By definition, the antecedent probability doesn’t change after the fact.
Here’s the relevant principle: To explain why p rather than not-p, you must cite something that’s true in the case of p but not true in the case of not-p. Indeterminists say that the antecedent probability is the same whether the atom ends up decaying or not. So the antecedent probability can’t explain why the atom decayed rather than not.
