In this essay, an attempt is made to formalize the idea of analogy in a way which is as faithful as possible to Thomas Aquinas’ theory of analogy. To accomplish this, we must first present Aquinas’ theory of analogy as it appears in his main works; we then express the contents of Aquinas’ theory of analogy using Leśniewski’s Ontology, a symbolic language which is both rigorous and true to the spirit of Aquinas’ philosophy. In doing this we present definitions and (...) theorems lying outside the scope of Leśniewski’s Elementary Ontology and we demostrate that the notion of “to be the definition of” is not an extensional functor. (shrink)