  I provide a simple derivation of the Born rule as giving a classical
probability, that is, the ratio of the measure of favorable states of the
system to the measure of its total possible states. In classical systems, the
probability is due to the fact that the same macrostate can be realized in
different ways as a microstate. Despite the radical differences between quantum
and classical systems, I show that the same can be applied to quantum systems,
and the result is the Born rule. This works only if the basis is continuous,
but all known physically realistic measurements involve a continuous basis,
because they are based eventually on distinguishing positions. The continuous
basis is not unique, and for subsystems it depends on the observable. But for
the entire universe, there are continuous bases that give the Born rule for all
measurements, because all measurements reduce to distinguishing macroscopic
pointer states, and macroscopic observations commute. This allows for the
possibility of an ontic basis for the entire universe. In the wavefunctional
formulation, the basis can be chosen to consist of classical field
configurations, and the coefficients $\Psi[\phi]$ can be made real by absorbing
them into a global U(1) gauge. For the many-worlds interpretation, this result
gives the Born rule from micro-branch counting.
