I find the idea that an actual infinity is impossible very counterintuitive, but sometimes arguments can establish something very counterintuitive. Actually, even if the following argument does not show that actual infinities are impossible, it will, I think, show that one cannot make any probabilistic inferences in an infinite multiverse. And that's an interesting conclusion in its own right. (That said, I have some technical qualms about the argument that I can't articulate.)
Begin with the Parity Principle: If it is almost certain (i.e., if it has probability one) that (a) the basic properties Q and R have exactly the same distribution at t1, and that (b) x is a substance existing at t1 and a member of basic kind K, and no other information is available about x, then the probability that x has Q is equal to the probability that x has R. (Here, I allow such probability values as "undefined", "inscrutable" as well as intervals, vague values.)
For my argument I will need the assumption that it is possible to have indiscernibles--objects that have all the same basic properties. I actually think this assumption is false, but I am hoping that this assumption can be relaxed.
The possibility of indiscernibles and the Parity Principle are going to be the only potentially controversial assumptions in the argument. If you trust me on this point, you can stop reading the argument, and just argue against these two assumptions. Though you might want to read on to see how exactly I understand the "same distribution" condition in the Parity Principle.
