  In fact, Hume criticised the whole conception of substance for lacking in empirical content: when you search for the owner of the properties that make up a substance, you find nothing but further properties. Consequently, the mind is, he claimed, nothing but a ‘bundle’ or ‘heap’ of impressions and  ideas – that is, of particular mental states or events, without an owner. This position has been labelled bundle dualism, and it is a special case of a general bundle theory of substance, according to which objects in general are just organised collections of properties. The problem for the Humean is to explain what binds the elements in the bundle together. This is an issue for any kind of substance, but for material bodies the solution seems fairly straightforward: the unity of a physical bundle is constituted by some form of causal interaction between the elements in the bundle. For the mind, mere causal connection is not enough; some further relation of co-consciousness is required. We shall see in 5.2.1 that it is problematic whether one can treat such a relation as more primitive than the notion of belonging to a subject.