Nicomachean Ethics

...and according to their being and formal reason, and are most greatly abstracted from matter itself, such as the substances which are called separate, and especially the most glorious God himself, who is most pure and stands entirely apart from all matter; finally, there are certain beings which in part can be separated and in part cannot be separated from matter. These, moreover, are mathematical things, such as the triangle, the circle, and others of the same kind, which must exist in matter and in some subject according to their being. They are separated, however, according to their definition and formal reason. And to persist in the same example, a triangle or a circle cannot exist [except] in some matter or subject, whether of wood, iron, silver, gold, or the like; yet it can be separated and abstracted according to its formal reason, since neither the subject in which it exists nor any sensible matter enters into its definition in any way. This seems to happen to such things for this reason, as I think: because they do not determine for themselves any proper subject or proper matter, as natural forms do. Wherefore, if a geometer defines a triangle, he will show it to be a plane figure contained by three straight lines. Again, if he wishes to assign the reason and definition of a circle, he will say that a circle is a plane figure surrounded by a single line, which is called the circumference, in the middle of which is a point from which all straight lines drawn to the circumference are equal. And in these definitions, neither iron, nor wood, nor anything at all of those things in which either a triangle or a circle can exist is included. Since, therefore, those things which are, or the things themselves, are divided threefoldly, and speculative philosophy is said to be about universal things and beings, it is deservedly distributed into three parts, as we have said: into natural, mathematical, and metaphysical, which is usually called first philosophy. I omit, however, those parts into which natural or mathematical science [is divided...]

God is a separate substance.