 In general, Locke’s particular substances are not ontologically basic, because their essences are nominal, though this is not so clearly true for sortals naming biological kinds. Locke’s conceptualism about such substances makes most of the tests irrelevant, which are cast within a realist framework. Locke’s ‘particular substances’ are kinds that are individuated conventionally, and so are not real substantial kinds, by Aristotelian standards. But Locke is not without substance other than substratum, for his real essence—that is, the atoms and their structures—are his real substances, and what we say above about the substantiality of atoms also goes for Locke.