Aristotle identifies three elements in substance: tode ti or “a ‘this'”; koris or separability and being to ti aen einai, which Latin Scholastics named by turning the verb “exist” into a noun: essentia. The first two criterion are pretty basic: substance is something you can point at and which can be divided from any other individual. The last criteria is more obscure but also more fundamental – in Aristotle it seems to end up meaning form (and whatever else form needs). This is why Aristotle in De anima II will describe soul as substance (along with what the soul needs) and why substance pure and simple is what is form pure and simple, i.e. the separate forms that Aristotle concludes Metaphysics VII with or that he proves must exist from motion in Metaphysics XII. When he considers nature as such he knows that form alone is not a possibility, and so in that discourse essentia is form and the matter that form needs, though even here he insists that nature is more form than matter (the inconsistencies that some commentators have tried to hang on Aristotle on this point can be cleared up by noting that speaking about to ti aen einai as cuch is not at all the same thing as speaking about it in nature).
Now if we raise the question of what “substance dualism” of the human person would mean under this sort of scheme many questions immediately vanish and other ones come into sharper focus. If we limit our account of substance to the first two criteria we might get our taboo Cartesian dualism, but this becomes untenable as soon as the third criterion enters in, though for incompossible reasons. If the person were a separated substance that for some unknown reason happened to be tied to a body at the moment then we wouldn’t have substance dualism – the substance would be the soul pure and simple. On the other hand, when form needs something other than form the form is not a substance though it is still “more substance” than the other thing. Under this description form is a part of substance as opposed to a substance while it is uniquely comparable to the whole. Either way, there is nothing deserving of the name “substance dualism” in Aristotle’s description of substance. What we have instead is a much more interesting description of the person as not his soul to the extent that he is a natural thing, i.e. to the extent that he is mobile in Aristotle’s sense and therefore an unactualized self.
(And now to shift gears entirely to a highly speculative first-draft eschatology)
Christian anthropology and eschatology beg to be grafted onto this sort of scheme and to provide it with a logical development. The separation of soul from the body is the moment when further unactualized possibilities of either perfection or degradation cease. Human virtue can longer be improved or lost and vices can neither plunge into further degradation nor be repented of. Note carefully that this is true so far as we are talking about the unactualized possibilities that are properly natural, i.e. that fall within the ambit of the cosmos as it is presently constructed. All that the person has achieved or failed to achieve as a natural being has come to a close and stands in need of a definitive judgment. But there is no need to describe the soul as looking back to nature as the domain of unactualized possibilities. A separated soul is not in a state a privation like a mutilated animal since mutilation differs from death by the continued function of some integral or essential part, and if any body part is integral or essential to the separated soul then the very notion of a separated soul is contradictory. The separated soul is a complete person but is no longer an ens mobile. He no longer looks back to the domain of unactualized natural possibilities nor has any ontological orientation to it.
The resurrection of the body is not necessary because the soul has some unsatisfied longing to return to the cosmos but because the person shares a common destiny with the cosmos that is itself destined to be remade and born again. Just as death marks the point where all cosmic possibilites of the person cease and are judged, the apocalypse of Christ marks the point where the possibilities of the cosmos come to an end and are reborn without judgment into beatitude (the cosmos, after all, never sinned). Man is remade and resurrected only in the context of the definitive rebirth of a new cosmos – “new heaven and new earth” – though he must enter into it after a judgment his actions as a member of the cosmos that has passed away. In his own passage from the cosmos, he was judged only on his own virtues and vices, but in the general judgment he must be judged for all the effects his just or unjust actions had on others, whether human or non-human.