  According to quantum mechanics, statements about the future made by sentient
beings like us are, in general, neither true nor false; they must satisfy a
many-valued logic. I propose that the truth value of such a statement should be
identified with the probability that the event it describes will occur. After
reviewing the history of related ideas in logic, I argue that it gives an
understanding of probability which is particularly satisfactory for use in
quantum mechanics. I construct a lattice of future-tense propositions, with
truth values in the interval $[0,1]$, and derive logical properties of these
truth values given by the usual quantum-mechanical formula for the probability
of a history.
