The probability of a proposition p equals the probability that p is true. I have argued that this principle refutes open future views. It’s interesting that it also refutes the many-worlds interpretation of quantum mechanics.
Suppose that I have prepared an electron mixed spin state 2−1/2|↑⟩+2−1/2|↓⟩ and we are about to measure whether the spin is up or down. The Born rule says that I should assign probability 1/2 to each of the two possible measurements. But by the many-worlds interpretation, the world splits into two (or more—but I will ignore that complication as nothing hangs on the number, or even the number being being well-defined) branches: in one an electron in a spin-up state is observed and in the other one in a spin down state is observed. Now consider these two propositions:
I will observe a spin-up state.
I will observe a spin-down state.
Given the many-worlds interpretation, metaphysically reality is symmetric with respect to these two propositions, as reality includes branches with both observes and with the observer standing in the same relationship to me. Hence, either both are true or neither is true on the correct reading of the many-worlds metaphysics: both are true if the observer in both branches counts as me, and otherwise both propositions are false. If the correct reading of the metaphysics is that both are true, then the probability of each being true is 1, and hence by the principle I started the post with, the probability that I will observe a spin-up state is 1 and so is the probability that I will observe a spin-down state. If the correct reading of the metaphysics is that neither is true, then the probability of truth for each will be 0, and hence the probability of my making either observation is 0.
So, the probabilities of (1) and (2) are 0 or 1. In neither case are they 1/2, which is what the Born rule stipulates.
This seems to me to be a stronger argument than the more common argument against the many-worlds interpretation that all branches should have equal probability, and hence would violate the Born rule in cases where the quantum state has unequal weights. For the usual argument depends on indifference, which is a dubious principle.