 Natural philosophy (as contrasted with metaphysics and mathematics) in the Aristotelian tradition is the science of physical substances in the sublunary world. The aim of natural philosophy is knowledge of causes, and for the Aristotelian tradition, this includes knowledge of essences. Locke’s above definition of real and nominal essences is given as a critical response to the Aristotelian influenced Scholasticism of the late medieval and early renaissance natural philosophy. Locke’s distinction between real and nominal essences appears to be in reference to the Aristotelian distinction between real and nominal definitions. A real definition, according to the Aristotelian tradition, is one that accords with the natural hierarchy and identifies the essence of the species or genus being studied, e.g., ‘man is the rational animal’ (reason being the essence of the species, animal being the genus). A nominal definition, by contrast, is a pseudo-definition that identifies the members of the species or genus, but fails to capture the essence, e.g., ‘man is the featherless biped’. A science of nature, for Aristotle and the Scholastics, consists of real definitions and not mere nominal ones. While Locke agrees that a science requires knowledge of essences, and thus of species, he differs from the Aristotelians by arguing that there are no real definitions of substances, but there are of moral and mathematical concepts (see Ayers 1996, Locke, vol. II, pp. 18–30). This account of real essences, then, is crafted to be capacious enough to accommodate both the Aristotelian and corpuscularian versions on offer. 