ABSTRACT
Philosophy.—Identity is generally understood to mean numerical identity, thing a is numerically identical to thing b if a and b are one and the same thing. Numerical identity must be distinguished from qualitative identity, which applies to two distinct things that share all of their characteristics except their spatiotemporal properties. This identity relation satisfies the indiscernibility-ofidenticals principle, which stipulates that if a is identical to b, a and b have the same properties. Reciprocally, the identity-of-indiscernibles principle posits that if two things share the same properties, they are identical. Gottfried Leibniz’s law states the equivalence of these two principles.
