1. In the third article, Thomas asks whether God is identical with his Essence or Nature, or merely an individual instance of that Essence. We can perceive at this point a progression in the articles of this question. Most things we observe are bodies, and so the most natural assumption would be that God is a body. Failing that, God must at least be the sort of thing that participates materially in a form. Once that has been excluded, Thomas begins to introduce us to the profound strangeness of the Godhead, by showing that God and his Godhead are identical.
2. Against the identity of God and his Essence (Godhead or Divinity), St. Thomas proposes two objections. First, that because we say that Divinity is "in" God, and nothing can be "in" itself (since this would seem to confuse the categories of substance and accident), God cannot be identical with his Divinity.
3. The second objection is noteworthy, because it contains the first appeal in the Summa to the principle "omne agens agit sibi simile", Every agent acts to its own likeness. This means that in acting every agent imposes its form on other things, to the extent that the perfection of its act and the receptivity of the thing receiving the act allow, and therefore that the effects of every act bear some likeness to the agent which caused them. The objector cites this principle, and the fact that God made man in his image, to conclude that if in man essence and supposit (i.e. the particular individual, "this man") are distinct, they must also be distinct in God.
4. For the Sed Contra, St. Thomas cites Christ's discourse at the last supper in John, "I am the Way, the Truth, and the Life." Since Christ identifies himself with life itself, Thomas sees this as warrant for identifying God and his Godhead. He elaborates beautifully in his comment on this verse (Cf. Commentary on John, paragraph 1868).
5. The argument in the corpus is fascinating, and somewhat difficult. The key to Thomas’s reasoning is that forms which enter into composition with matter are individuated by that matter, so that in such forms the individual supposit and the essence of the species (of which the supposit is a member) are always distinct.
6. For example, an apple is such a thing as could not exist immaterially, since it belongs to the form to be united with matter: the removal of matter from the form would be the destruction of the form as well. Thus every apple is distinct from “apple-ness”, because each apple supplies the material principle lacking in the form, and thus exceeds the mere form in its determinacy and reality. As Thomas puts it, “the essence or nature includes in itself only those things which fall into the definition of the species.” This is to say that in particular species of things, the essence of the species is limited to the form which constitutes the species. The particular qualities of the matter to which this essence is added, and the particular accidents which might be added to the essence once it is received in an individual, are not part of the essence. Thus in individual material things there is always more than the essence of the species to which they belong.
7. Among those forms that are not received in matter, the case is rather different. In this case the matter is not needed to receive the form, since the form has actuality in itself apart from being individualized in a particular recipient. Furthermore, such a form does not need to be individualized, because it is distinct by being itself, and not by being instantiated in this or that quantity of matter. God, however, is clearly one of these sorts of forms, and therefore in him essence and supposit are not distinct. Thus God is the same as his Godhead, his Life, and whatever other forms can be truly predicated of him.
8. In response to the first objection, Thomas clarifies that although God’s “divinity” is spoken of as though distinct from himself, this is merely to ease the way for the human mind.
9. To the second objection, Thomas answers that the likeness of the creator in the creature is imperfect and diverse. This suffices to explain the difference.
Outline of Article:
—Godhead is said to be "in" God.
—Humanity, the image of God, differs from individual humans.
—I am the Way, the Truth, and the Life.
—Among material things form is received or super-added to the thing. The matter receives additional actuality from the form, but the form requires the matter in which to be actualized. But in God, the form or essence subsists, and therefore is individualized not by its reception in matter, but simply by being itself.
—This language is based on an analogy to human nature.
—The likeness between creator and creature is imperfect.