As near as I understand the question, let me answer with a bit of historical narrative.

Peter Abailard was interested in the question of universals. His definition was that it is the kind of thing that can be predicated to many things (e.g., man - man is a universal because it can be applied to every man). Given the logical definition, he went on to ask the nature of universals (n.b., this is not a question for logic, but for philosophy). His logic professor (William of Champeaux) was something of a realist. That is, universals were not merely concepts of the mind, but real things--substances--found in things. Plato and Socrates are both men precisely because there is something in common between them: their humanity, and this humanity must be just as real as any thing else we really talk about.

Abailard pointed out the fallacy in that perspective immediately, which I think goes a long way in answering your friend's question. If human nature is a real thing--a substance--that is present in both men, then it is only partly present in both men; but if it is only partly present, then neither man is truly man, but only partially man. If humanity is entirely present in one, though, it cannot be present at all in another, which would make one a man and not the other. He was right thus far, but then he took a wrong step: if humanity can neither be partly nor completely found in any man, then it is not something; and that which is not something is nothing.

Now William tried to find a middle ground for his position, but ultimately failed for the reason that all strict realists must fail. Abailard's argument was correct, insofar as it went. Universals cannot be real substances, for if they are--as real things--they are either in things wholly or partly; if wholly, they are in nothing else, and if partly, then the thing in which they are found are not fully that thing. But that would prove no comfort to Abailard. For his own view was equally flawed. For if "humanity" is really nothing at all, then there is nothing in a man that corresponds with our word "man." Thus, we see Abailard's reasoning fail for the same reason all nominalism fails -- if there is nothing real (in some sense) corresponding to our words, then our words have no real meaning. In other words, having proven universals are not substances, Abailard had to show how we could justifiably attribute them to things if they do not really exist at all--that is, if there is nothing in the thing to which we are attributing them to which they can correspond. He failed to accomplish his impossible task, precisely because he had not taken the road Aristotle did.

So perhaps Aristotle wasn't a novice missing such things after all . . .

Or perhaps I've completely misunderstood your question, and as such, would point you to the better educated Aristotelians that hang around here.

I would break that question up into two:

1. Can form exist without matter?
2. Do we know that form really does exist without matter?

Your question could be taken either way. As to the former, to answer 'no' would require demonstrating that form, in virtue of being form, must necessarily be the form of a body. But I don't see why that is the case at all. Form is the active principle of a body, true, but I don't see anything logically inconsistent with saying that form--as actuality--can exist without matter, since potentiality presupposes act. Aquinas is right, of course, when he points out pure potentiality cannot exist, because then it wouldn't be actual.

Rather than trying to prove (1) to be true, I'd just ask him for any reason to think it wouldn't be. I mean, since actuality can exist without potentiality, I see no reason form can't exist without matter.

As to the second question, the proof of spiritual entities proves as much. It's easy enough to prove the soul really exists. It's easy enough to prove God exists. If you are a Christian, both of these are still easier, and other things such as angels obviously exist. So an affirmative answer to (1) is necessary to believe in the existence of pure form; an affirmative answer to (2) is sufficient.

But beyond all that, I have to confess, why is your friend having such difficulty with the idea that form can exist without matter?

Angels are pure form (at least according to St. Thomas).

Your friend is only looking at the Categories, which is a logical work and not hence an attempt to solve metaphysical questions?

Anyhow. Substance in its primary sense is that which is neither said to be in something or said of something, but other things are said to be in or of it. Secondary substance is said of primary substance, it is a derivative use. Your friend is trying to make the Categories try to prove too much. Here we are dealing with a logical distinction.


Maybe part of the difficulty is the translation "present in" Aristotle means by "in" "not as being a part, but as being unable to be separated from that which it is in" Now humanity itself is something of a universal predication said of many substances, hence humanity falls under the summa genus of substance as a secondary substance, a species. Just as color is a genus of quality, and whiteness a species of color, but this white is a particular accident existing in this substance.

One doesn't have to answer the questions of universals, or how humanity exists in this or that man, or how this white that exists in this thing relates to the universal whiteness. This is merely the categories, we are delving into the way we speak about things and clearly there is a distinction, at least in reason, between the particular and the universal, between the accident of white in me, and whiteness and between this man and humanity itself.

Is the soul pure form in the same way as angels?

No, as the soul is the form of a material substance, and hence is an element of the whole substance, and not a whole substance by itself

The human soul separated from the body does subsist, and can be called an individual substance like the angels, but with some qualifications, as it remains in its essence the soul of this body and retains its disposition to be such, hence it is imperfect in its mode of existence