Abstract

This paper sketches a history of definition of identity from the Aristotle’s Topics down to the modern set theory. The author tries to explain particularly: first, how the transformation of the concept of predicate at the end of the nineteenth century made it necessary to revise the leibnitian definition of the identity of individuals; secondly, why Dedekind, Peano, Schröder, etc. made, between two possible definitions of identity of predicates or of sets, a choice which later made it necessary to postulate in set theory the axiom of extensionality.