  Discussion of real essence often focuses upon the real essences of material substances, and in that case we say that the real essence is the causal ground of the substance’s perceivable properties. As will be discussed in a subsequent section, many commentators interpret Locke as identifying a material substance’s real essence with some subset of its constituent corpuscles’ primary qualities, an identification presuming that Locke accepts the corpuscular hypothesis (e.g., Osler 1970, p. 12; Mandelbaum 1964, p.1). According to another interpretation, the real-nominal essence distinction is metaphysical, and thus more fundamental than the primary-secondary quality distinction, which is a physical distinction, belonging as it does to a particular physical theory, the corpuscular hypothesis. Yet leaving that debate aside for the moment, we may note from the first sentence of the above-quoted passage that Locke does not restrict the notion of real essence to substances. This means that we can speak of, say, the real essence of a triangle, understanding it as that which grounds the triangle’s qualities, making it what it is.