I distinguished in earlier posts two conceptions of primary substance and argued that Aristotle, in the Categories, means by a 'primary substance' (roughly) an instance of a nature.
But corresponding to this is a distinction, too, in senses of 'this'. We know that, in the Categories, Aristotle says that a primary substance is a 'this' (tode ti). But this 'this' might mean either of two things:
(i) A particular. To be contrasted with a universal. A particular is something which, if it is perceptible, at least, has definite existence in space and time. It exists in a place and for some determinate time. To say that something is a particular in this sense is to group it with all and sundry other particulars: Socrates, the Eiffel Tower, Jupiter, the water in my glass.I think it can be shown, from definite texts in the Categories, that there, by tode ti, Aristotle means (ii) and not (i).
(ii) An individual. To be contrasted with a kind or sort. Something is an individual always of some kind or nature (an individual horse, an individual human). To say that something is an individual, is to say that it is one of (potentially) many other instances of that kind. To say that something is an individual is to group it implicitly with other individuals of the same kind.
But here's a neat argument by Mary Louise Gill that Z3 shows the same thing. It is from her contribution to the forthcoming Blackwell Companion to Ancient Philosophy, in a chapter called "First Philosophy in Aristotle":
Z1 also mentions thisness. Aristotle said that being is said in many ways. In the first place it signifies what something is (ti esti) and a this (tode ti), and then the other categories (1028a10-13). Thisness is a distinguishing feature of substance. Scholars often take thisness to indicate particularity. Although Aristotle's use of the phrase in the Categories supports this claim (Cat. 5, 3b10-18), its application is probably not so restricted. Reflection on the phrase itself suggests another relevant factor. The phrase can be literally translated in two ways: "some this" or "this something". In either case one term presumably indicates a kind, and the other something that falls under that kind. The item marked off could be either a particular that falls under a kind (some horse, this horse) or a determination of a wider kind (a sort of horse, this sort of horse). That thisness does not simply mean "particular" seems assured, since matter in Z.3 fails the test. A bare subject is surely a particular. Matter's lack of thisness is rooted in a different fault. As literal translations of tode ti suggest, something counts as a this only if it is something determinate or particular that falls under a kind. Matter in Z.3 may be a particular, but it does not fall under a kind, since it has no categorial being, and a fortiori no substantial being, in its own right.The ultimate subject discussed in Z.3 is a particular; yet (Aristotle insists) it is not a 'this'; thus 'thisness' , for Aristotle, does not indicate something's being a particular.