How did I come to presume that a primary substance, in the Categories, was an ordinary thing?
I believe: from my bringing to that text the view that propositions of the form, "a is F", are atomic, where a is a proper name: it is then natural to interpret Aristotle's claim, that primary substances are neither said of, nor exist in, anything, as corresponding to the linguistic mark that proper names are not predicated of anything (except to assert an identity). Proper names, then, would seem to name primary substances. But proper names indicate ordinary things. Thus primary substances are ordinary things.
I was encouraged in this line of thought by Ackrill's commentary, because he simply presumes without argument that primary substances are individuals, and he illustrates what Aristotle says about substances, by sentences using proper names--when Aristotle never gives such examples himself. The following text provides a good example of this. What Aristotle says is:
It is a characteristic common to every substance not to be in a subject. For a primary substance is neither said of a subject nor in a subject. And as for secondary substances, it is obvious at once that they are not in a subject. For man is said of the individual man as subject but is not in a subject: man is not in the individual man. Similarly, animal also is said of the individual man as subject but animal is not in the individual man. Further, while there is nothing to prevent the name of what is in a subject from being sometimes predicated of the subject, it is impossible for the definition to be predicated. But the definition of the secondary substances, as well as the name, is predicated of the subject: you will predicate the definition of man of the individual man, and also that of animal. No substance, therefore, is in a subject (3a7-20, Ackrill).Note that there is no mention here of individuals such as Socrates or Callias. The text itself gives us no reason to think anything other than that, if Aristotle were to support what he is claiming by appeal to language, he would examine sentences of the form, "Man is in the individual man" or "'Rational animal' is said of the individual man."
Yet Ackrill comments as follows, speaking as though 'Callias' names a primary substance:
Why is it 'obvious at once' that secondary substances are not in primary substances? It is not that they can exist separately from primary substances (2a34-b6). Nor does Aristotle appear to rely on the fact that a given secondary substance can exist separately from any given individual, that there could be men even if Callias did not exist, so that the species man can exist separately from Callias and is, therefore, not him. Aristotle seems rather to be appealing to the obvious impropriety in ordinary speech of saying such things as 'man is in Callias'. ...And the rest of the commentary is similar.
One cannot say 'hero is in Callias' or 'father is in Callias'; but if Callias is a hero and a father the definition of 'hero' and 'father' can also be predicated of him...
I wonder, too--as regards this particular passage--whether what Aristotle claims isn't indisputably obvious, when we take a primary substance to be (as we claim) an instance of a nature, so that 'this particular man' means 'this instance of human nature'. For then: when we say "This particular man is white", we are supposing, clearly, that the white is one thing, and that this particular man is another thing. But, when we say "This particular man is a man", it's obvious that it is not the case that this particular man is one thing, and that what is indicated by 'man' is another thing: we wouldn't want to say, in that sense, that what is indicated by 'man' exists in this instance of human nature.
On the other hand, if the primary substance, 'this particular man', is not a human being, but rather the substance of Callias, then it is not, indeed, obvious that 'man' is not somehow existing in Callias.
That is to say: Ackrill's failing to find obvious, what Aristotle finds obvious, arises from his mistake of taking primary substances to be ordinary things.