In describing the account of the εἷδη in the Phaedo, Burnet says, ‘they are explained in a peculiar vocabulary which is represented as that of a school. The technical terms are introduced by such formulas as “we say”’. Similarly Taylor has written of the ‘characteristic technical nomenclature’ used in the dialogues, of the ‘technicalities’ of the theory of εἷδη, of ‘the technical phrases of the Phaedo’ The validity of such language has been taken for granted by both these and many other Platonic scholars. But the assumption which it represents—that Plato employed certain words in a significance peculiar to his use of them—carries such wide implications for the history and interpretation of his philosophy that it can hardly be accepted without further investigation. In this article I shall examine the evidence in fifth- and early fourth-century literature about these words which Plato or Socrates is alleged to have transformed into ‘technicalities’: first, εἷδος and ἰδ⋯σ, which I shall assume to be more or less synonymous; second, the various terms used to describe the relation between εἲδη and particulars—,μετέχειν, κοινωνεἶν,παρεἶναι, ⋯νεἶναι,μιμησις,⋯μοιωαις, and so on.