  The second salient feature is the dominant scientific theory of his day, the new science’s corpuscular hypothesis. As defined for the purposes of this article, the corpuscular hypothesis (i) takes observable bodies to be composed of material particles or corpuscles, (ii) takes impulse (action by surface impact) to be the primary if not the sole means of communicating motion, and (iii) attempts to reduce qualities at the level of observable bodies, such as color, to the primary, that is, inherent properties of the particles composing those observable bodies. In what may be called its orthodox version (“pure mechanism”, as Ayers (1981, p. 212) calls it) the corpuscular hypothesis restricts those inherent properties to size, shape, number, and motion, and holds that all other qualities and operations are explicable in terms of that restricted set of properties.  The orthodox version thus  implies a proviso of contact action—that bodies causally interact only locally, by impact, such that unmediated action at a distance is denied. (Although a number of commentators use the terms ‘corpuscular hypothesis’ and ‘mechanism’ interchangeably, distinguishing them has certain benefits. For instance, it permits us to classify Newton as a corpuscularian theorist of some stripe, and to do so without engaging the debate about whether he adhered to the contact action proviso. The definitions given here also agree largely with those in the entry on   John Locke.) Plenist and atomist versions of the corpuscular hypothesis may be distinguished. Plenist theorists deny the void and assert a plenum of matter, as Descartes does by identifying matter with extension. Such theorists may speak of particles, but their particles are not atoms, being infinitely or at least indefinitely divisible. Atomist theorists, by contrast, accept the void and take the particles or corpuscles comprising compound bodies to be indivisible, or at least probably so. Since Locke’s sympathies are clearly with the atomist version, the term ‘corpuscular hypothesis’ shall refer to that version throughout this article, unless indication is given otherwise. Central theses of the Essay are developed in close conjunction with the corpuscular hypothesis—most notably, the distinction between real and nominal essences, which is developed in connection with the distinction between primary and secondary qualities associated with corpuscular theorists, including Locke’s mentor, Robert Boyle. Yet since Locke often treats the hypothesis with skepticism, its status and purpose are a source of controversy.