  This article explores the following methodological principle for theory
construction in physics: if an ontological theory predicts two scenarios that
are ontologically distinct but empirically indiscernible, then this theory
should be rejected and replaced by one relative to which the scenarios are
ontologically the same. I defend the thesis that this methodological principle
was first articulated by Leibniz as a version of his principle of the identity
of indiscernibles, and that it was applied repeatedly to great effect by
Einstein in his development of the special and general theories of relativity.
I argue for an interpretation of the principle as an inference to the best
explanation, defend it against some criticisms, discuss its potential
applications in modern physics, and explain how it provides an attractive
middle ground in the debate between empiricist and realist philosophies of
science.
