With the distinction of essence and existence now in hand, it should prove enlightening to compare, more closely, the metaphysical structure of one grade of being with that of another, the material creature with the spiritual, and these with God.
a) Material substances have a twofold composition: of matter and form in their essence, of essence and existence in their concrete reality. Form actualizes and determines matter; together they constitute the composite essence of the material substance, which is further composed with its ultimate actuality, existence. The principle of individuation for such substances is matter terminated by quantity, technically signate matter. To them, accordingly, applies this arrangement:
what is (quod est) = the individual
by which it is (quo est) = essence
by which it is (quo est) = existence
b) Spiritual substances are simple in essence, that is, not composed of matter and form. The composition of essence and existence does, however, appear in them, as of course in every creature. Regarding individuation, this the subsistent (or pure) form has of itself - matter, obviously, cannot be the principle of it here. 10 Thus, in the spiritual substance,
what is (quod est) = essence
by which it is (quo est) = existence
Essence, then, as appears from the above, can be considered as a subject (quod), this in the material substance, or as a formal principle (quo), speaking of the spiritual substance; whereas existence is always a formal principle, albeit in a very analogical sense.
c) Uncreated being, God, is absolutely simple. God, this means, is not distinguishable into subject of existence and existence of the subject, that is, into essence and existence. His existence (esse) is self-subsistent and identical with his essence; moreover it is infinite, from not being limited by anything at all. It is also necessary, since God's being excludes the possibility of its not-being. God, in a phrase St. Thomas favors, is Ipsum esse subsistens, Subsistent Being Itself. Because the essence of God is to exist, some would have it that in God there is no essence. This is true enough if it means that his existence is not determined by a formal principle of any kind, but it is false in the sense that God's being has no nature at all, or is an indeterminate infinity.