Locke introduces his famous distinction between real and nominal essences with little fanfare halfway through the third chapter of Book III of the Essay Concerning Human Understanding. This distinction is his own invention. Terminologically, it is clearly intended to reflect the Scholastic distinction between real and nominal definitions, but, as Leibniz complains, it is an innovation of Locke's to use these terms to refer to essences. “It seems to me,” Leibniz writes, “that your way of putting things constitutes a very novel mode of expression. People have certainly spoken of nominal definitions, and 'causal' or 'real' ones, but so far as I know they have not until now spoken of essences other than real ones” (Leibniz [1765] 1981: 293). Locke's novelty is a matter of altering terminology belonging in a theory of language to a metaphysical use, rather surprisingly, in the middle of the portion of his Essay entitled “On Words.” This innovation of Locke's, despite its rather modest entry into his argument, has been treated by most commentators as both important and central to his theory. Genevieve Brykman, indeed, describes it as a “cornerstone” of his thinking (2001: 81). But the nature of this cornerstone and its role in Locke's metaphysics and in his theory of language have proved controversial.