I have been considering the question: Does Aristotle mean by a 'primary substance', in the Categories, an ordinary 'thing', or does he mean an 'instance of a nature'? Is it Socrates, or rather this human nature of Socrates, which serves as an example of a primary substance? I maintain: the latter. A primary substance is an ordinary thing of a certain kind less its determinate accidents.
An instance of a nature is of course a 'this', in the sense in which a 'this' may be distinguished from a sort or kind. Consider a sick man in bed, immobile, with parched lips. He sees a glass of water on his bedstand and says, "Water!". He may mean by this either:
"The sort of thing I want is water", orIf the former, then he would have used the word 'water' to indicate a secondary substance; if the latter, then he would have done so to indicate a primary substance. In the latter usage, 'water' signifies a 'this something' (tode ti).
"I want that water."
It is because words for kinds admit of this ambiguity, and can mean instances of kinds just as wells as the kinds themselves, that Aristotle makes the clarifying remark at Categories 3b10. That he has to clarify in this way shows, I argued in an earlier post, that he does not think of a primary substance as what would be naturally signified by a proper name. Thus primary substances are instances of kinds, not ordinary things. They are 'individuals' not 'particulars'.
"Scholars often take thisness to indicate particularity," M.L. Gill observed; and she seemed to accept that "Aristotle's use of the phrase in the Categories supports this claim (Cat. 5, 3b10-18)". But in fact we have seen that Aristotle's use of the phrase in that passage supports, rather, the view that tode ti indicates 'individuality', that is, being an instance of a kind.
A closer examination of the passage gives additional reason for thinking this. Consider in the passage the parallelism between Aristotle's use of the phrase tode ti and his use of the phrase poion ti. I'll highlight this in red for clarity's sake:
Every substance seems to signify a certain 'this'. As regards the primary substances, it is indisputably true that each of them signifies a certain 'this'; for the thing revealed is individual and numerically one. But as regards the secondary substances, though it appears from the form of the name--when one speaks of man or animal--that a secondary substance likewise signifies a certain 'this', this is not really true; rather, it signifies a certain qualification, for the subject is not, as the primary substance is, one, but man and animal are said of many things. However, it does not simply imply a certain qualification, as white does. White signifies nothing but a qualification, whereas the species and the genus mark off the qualification of the substance--they signify substance of a certain qualification. (One draws a wider boundary with the genus than with the species, for in speaking of animal one takes in more than in speaking of man.) (3b10-23, Ackrill)Let us suppose that, from this parallelism, we may conclude that Aristotle understands tode ti the way in which he understands poion ti.
Pa~sa de\ ou)si/a dokei= to&de ti shmai/nein. e0pi\ me\n ou}n tw~n prw&twn ou)siw~n a)namfisbh&thton kai\ a)lhqe/j e0stin o3ti to&de ti shmai/nei: a1tomon ga_r kai\ e4n a)riqmw|~ to_ dhlou&meno&n e0stin. e0pi\ de\ tw~n deute/rwn ou)siw~n fai/netai me\n o(moi/wj tw|~ sxh&mati th~j proshgori/aj to&de ti shmai/nein, o3tan ei1ph| a1nqrwpon h2 zw|~on: ou) mh_n a)lhqe/j ge, a)lla_ ma~llon poio&n ti shmai/nei, —ou) ga_r e3n e0sti to_ u(pokei/menon w3sper h( prw&th ou)si/a, a)lla_ kata_ pollw~n o( a1nqrwpoj le/getai kai\ to_ zw|~on:— ou)x a(plw~j de\ poio&n ti shmai/nei, w3sper to_ leuko&n: ou)de\n ga_r a1llo shmai/nei to_ leuko_n a)ll' h2 poio&n, to_ de\ ei]doj kai\ to_ ge/noj peri\ ou)si/an to_ poio_n a)fori/zei, —poia_n ga&r tina ou)si/an shmai/nei.— e0pi\ plei=on de\ tw|~ ge/nei h2 tw|~ ei1dei to_n a)forismo_n poiei=tai: o( ga_r zw|~on ei0pw_n e0pi\ plei=on perilamba&nei h2 o( to_n a1nqrwpon.
But how does he understand poion ti? It seems that here, as is common in Aristotle, a phrase of the form "____ ti" indicates that what is put in the blank has to be understood with a certain proviso. And the proviso that he wishes to add as regards poion is that it is peri\ ou)si/an; a secondary substance indicates a qualification that is tina ou)si/an. I'll highlight these phrases in blue.
What he means by this proviso is presumably that a term for a secondary substance ('animal'), unlike a term for a quality in the usual sense ('white'), is relevant for classification: what a secondary substance signifies serves to place something in a kind. When we call something an animal, we identify it as falling in a class of animals; but, in contrast, there is no meaningful class of 'whites' into which we sort something in calling it 'white'. Thus, words for secondary substances indicate a special and indeed privileged sort of qualification (poion ti).
But if this is what he means, then, given the parallelism between poion ti and tode ti, we should presume that, by tode ti, Aristotle understands a similar proviso, and the phrase then means "a this as regards substance", that is "a this as regards some basis for sorting". And if this is what it means in this passage (even if later it acquires a technical sense), then the phrase would precisely indicate what I have called an individual. It would pick out something as an instance relative to some kind.