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.

Berkeley's New Theory of Vision has had an excellent press. In the introduction to his edition, A. A. Luce presents it as a book that, from early on, received wide acclaim; a number of other authors have reinforced that impression. In the second part of his Dissertation (first published in 1821) Dugald Stewart makes this report: The solid additions, however, made by Berkeley to the stock of human knowledge were important and brilliant. Among these, the first place is unquestionably due to his New Theory of Vision; a work abounding with ideas so different from those commonly received, and, at the same time, so true, so profound and refined, that it was regarded by all but a few accustomed to deep metaphysical reflection, rather in the light of a philosophical romance than of a sober inquiry after truth. Such, however, has been since the progress and diffusion of this sort of knowledge, that the leading and most abstracted doctrines contained in it, form now an essential part of every elementary treatise of optics, and are adopted by the most superficial smatterers in science as fundamental articles of their faith.