Hume on what there is

Ontology was never Hume's main interest, but he certainly had opinions as to what there is, and he often expressed these in his philosophical works. Indeed it seems clear that Hume changed his ontological views while writing the Treatise, and that not just one but two different ontologies are to be found there. The ontology of Parts I, II, and III of Book I is more or less Lockean. There are minds and their operations and qualities. There are physical entities, bodily actions and qualities if not bodies over and above these. And there are further entities, called ideas by Locke and perceptions by Hume, that represent things other than themselves, both physical and mental, while existing in and being dependent upon minds. In Part IV of Book I, however, and especially in Sections 2 and 6, a new ontology appears, one that differs not only from the doctrine of the earlier sections of the Treatise but also from any that previous philosophers had held. According to this new ontology, there are only perceptions: all other sorts of things are absorbed by or reduced to these, or else simply eliminated. Berkeley had indeed assimilated bodies and the properties of bodies to perceptions, but he had kept minds as a distinct category of entity. Hume went the whole way, making everything perceptions. We might characterise this new ontology as trans-Berkeleyan, or call it, on account of its similarity to later doctrines so named, phenomenalism or neutral monism.