We must remember that a natural compound is the unity of its matter and form. Inasmuch as prime matter and substantial form are united, they exist as one thing, meaning that they constitute no reality that is separate and apart. We can construct an argument for such a thesis:
1.If natural things were not essentially the unification of prime matter and substantial form, then the natural thing would itself some third thing.
2.This third thing, as an actuality, would be the act of the unity of matter and form.
3.Substantial form, though, is the first act by which prime matter is rendered actual.
4.Therefore, this third act, supposedly constituting the unity of matter and form as an actual natural compound, would be an act apart from the substantial form.
5.It must be, therefore, an accidental form.
6.An accidental form, then, would constitute a natural thing in existence, which cannot be, since substantial form is prior to accidental form.
7.Therefore, the natural thing just is the unity of prime matter and substantial form.