 Substantive theories take the thought of a further component seriously, and, though they do not seem to be generally distinguished, there are two main conceptions of this extra element, which I will call the ‘thin particular’ and the ‘substratum’ conceptions. ‘Thin particular’ is an expression of D. M. Armstrong’s. It is the particular in abstraction from its properties. When considered with its properties it is a thick particular. The important point is that thin particulars really are particulars. ‘Bertrand Russell might have been a fried egg’: that is, the thin particular associated with one set of properties and, hence, with one thing, might have belonged to another. Different properties might have been hung on this hook. It is not so clear, by contrast, that a substratum of the kind Locke considers, or like Aristotle’s prime matter, is a particular in this sense. It is more like a kind of stuff—the substantiveness on to which properties are stuck. On this conception, the suggestion that the ‘piece’ of substratum or ‘prime matter’ that is here in object a, might always instead have been there in object b, seems to lack content.