Here's another passage from the Categories that suggests that Aristotle did not regard primary substances as 'atoms'. Call this Passage A:
We need not be disturbed by any fear that we may be forced to say that the parts of a substance, being in a subject (the whole substance), are not substances. For when we spoke of things in a subject we did not mean things belonging in something as parts. (3a29-32, Ackrill)This is perplexing, is it not? What does Ackrill say in his commentary, to clear it up? Nothing; he skips right over this passage.
mh_ taratte/tw de\ h(ma~j ta_ me/rh tw~n ou)siw~n w(j e0n u(pokeime/noij o1nta toi=j o3loij, mh& pote a)nagkasqw~men ou)k ou)si/aj au)ta_ fa&skein ei]nai: ou) ga_r ou3tw ta_ e0n u(pokeime/nw| e0le/geto ta_ w(j me/rh u(pa&rxonta e1n tini.
Nor does he refer to it, when he discusses the earlier passage, which we had looked at already, to which this one looks back. I'll quote it again. Call this Passage B:
Some are in a subject but are not said of any subject. (By 'in a subject' I mean what is in something, not as a part, and cannot exist separately from what it is in.) For example, the individual knowledge-of-grammar is in a subject, the soul, but is not said of any subject; and the individual white is in a subject, the body (for all color is in a body), but is not said of any subject. (1a24-27, Ackrill)About which Ackrill says:
Aristotle's explanation of 'in a subject' at 1a24-25 is slight indeed. One point deserves emphasis. Aristotle does not define 'in X' as meaning 'incapable of existing separately from X', but as meaning 'in X, not as a part of X, and incapable of existing separately from what it is in'. Clearly the 'in' which occurs twice in this definition cannot be the technical 'in' of the definiendum. It must be a non-technical 'in' which one who is not yet familiar with the technical sense can be expected to understand. Presumably Aristotle has in mind the occurrence in ordinary Greek of locutions like 'heat in the water', 'courage in Socrates'.I think not. Presumably Passage A is relevant at this point, and it indicates that the main non-technical usage of 'in', which Aristotle has in mind, is the way in which a part is in a composite whole.
And, given Passage B, wouldn't it be natural to understand Passage A as telling us that "we needn't be disturbed by any fear that we may be forced to say" that the soul, which is in this individual human (as a part), is not a substance, and that the body, which is in this individual human (as a part), is not a substance? That is: we are not hindered from analyzing this primary substance as compounded of two substances.
And then, finally, as Ackrill points out, observe that Aristotle has crafted Passage B in such a way as to leave open the possibility that the way in which the soul (or body) exists in the individual human is not such as to make it incapable of existing separately.