 Locke’s contribution is, therefore, three-fold. He brings to centre stage the question of whether properties require some substratum or bare particular to inhere in or belong to. He asserts that the substantial nature of the physical world is the unknown structure of atomic parts, not a substantial form which reflects our usual concepts. Third, he develops a theory of substance which is realist about particular objects and their properties, but conceptualist or conventionalist about our classifications, within the constraints that the facts about particulars and properties impose.