 The problem Lewis faced in the 1980s papers is that the best systems account of chance makes the Principal Principle either useless or false. Here is a somewhat stylised example. (I make no claims about the physical plausibility of this setup; more plausible examples would be more complicated, but would make much the same point.) Let t be some time before any particle has decayed. Let A be the proposition that every radioactive particle will decay before it reaches its actual half-life. At t, A has a positive chance of occurring. Indeed, its chance is 1 in 2n, where n is the number of radioactive particles in the world. (Assume, again for the sake of our stylised example, that n is finite.) But if A occurred, the best system of the world would be different from how it actually is. It would improve fit, for instance, to say that the chance of decay within the actual half-life would be 1. So someone who knows that the chance of A is 1 in 2n knows that A won’t happen.