  I show that frequentism, as an explanation of probability in classical
statistical mechanics, can be extended in a natural way to a decoherent quantum
history space, the analogue of a classical phase space. The result is a form of
finite frequentism, in which the Gibbs concept of an infinite ensemble of gases
is replaced by the quantum state expressed as a superposition of a finite
number of decohering microstates. It is a form of finite and actual (as opposed
to hypothetical) frequentism insofar as all the microstates exist, even though
they may differ macroscopically, in keeping with the decoherence-based Everett
interpretation of quantum mechanics.
