Socrates recognizes a distinction between formal and material definitional conditions. In this dissertation, I concentrate on the material conditions rather than the formal ones for two reasons: Socrates allows a great deal of syntactic flexibility, and many answers he regards as formally adequate resist classification by contemporary standards. I argue that Socrates is committed to four material adequacy conditions in answers to "what is F-ness?" He is committed to the extensional equivalence condition, that the definiens picks out all and only instances of the definiendum, the property sameness condition, that the definiens is a property identical to the definiendum, the explanation condition, that the definiens explains instances of the definiendum, and the semantic completeness condition, that statements about F-nesses involving evaluative predicates do not change their truth-value, regardless of context.

It is well-known feature of a number of Plato's dialogues that when Socrates asks his infamous, "what is F-ness?" question, where "F-ness" stands for "virtue" or "piety" or "courage" or "beauty" or "temperance, " no one gives him the answer he wants. This may lead one to believe even that Socrates has no adequate answer in mind. Nevertheless, by examining Socrates' arguments, his explanations of what he wants, and his model answers, one can specify the conditions an adequate answer to "what is F-ness?" must meet.