The soup thickens. We haven’t learned much about God yet, other than He exists, is not composite, is outside time, is pure actuality and so forth. To go further, we need to expand our vocabulary and introduce the idea of essence. Well summarized by The Catholic Encyclopedia (CE), this is “the radical or ground from which the various properties of a thing emanate and to which they are necessarily referred. Thus the notion of the essence is seen to be the abstract counterpart of the concrete entity; the latter signifying that which is or may be [(in actuality, in potential)], while the former points to the reason or ground why it is precisely what it is.”.
Chapter 21: God Is His Own Essence
1 FROM what has been laid down we are able to conclude that God is His own essence, quiddity or nature.i
2 In everything that is not its own essence or quiddity there must needs be some kind of composition: for since each thing contains its own essence, if a thing contained nothing besides its own essence, all that a thing is would be its essence. Therefore if a thing were not its own essence, there must be something in it besides its essence: and consequently there must be composition therein. For which reason the essence in composite things has the signification of a part, as humanity in a man. Now it has been shown ^1 that in God there is no composition. Therefore God is His own essence.ii
3 Again. Seemingly that alone which does not enter into the definition of a thing is beside the essence of that thing: for a definition signifies what a thing is. Now only the accidents of a thing do not enter into its definition: and consequently only accidents are in a thing besides its essence. But in God there are no accidents, as we shall show further on. Accordingly, there is nothing in Him besides His essence. Therefore He is His own essence.iii
4 Moreover. Forms that are not predicated of subsistent things, whether the latter be taken universally or singly, are not single per se subsistent forms individualized in themselves. For we do not say that Socrates, or man, or an animal is whiteness, because whiteness is not singly per se subsistent, but is individualized by its subsistent subject.iv Likewise natural forms do not per se subsist singly, but are individualized in their respective matters: wherefore we do not say that this individual fire, or that fire in general is its own form. Moreover the essences or quiddities of genera or species are individualized by the signate matter of this or that individual, although indeed the quiddity of a genus or species includes form and matter in general: wherefore we do not say that Socrates, or man, is humanity.v Now the divine essence exists per se singly and is individualized in itself, since it is not in any matter, as shown above. Hence the divine essence is predicated of God, so that we say: God is His own essence.vi
5 Further. The essence of a thing is either the thing itself, or is related to it in some way as cause: since a thing derives its species from its essence. But nothing can in any way be a cause of God: for He is the first being, as shown above.vii Therefore God is His own essence. Again, that which is not its own essence, is related in respect of some part of itself to that essence, as potentiality to act: wherefore the essence is signified by way of form, for instance humanity. But there is no potentiality in God, as shown above, therefore it follows that He is His own essence.viii
iQuiddity: the whatness (as Kreeft says), “the essence that makes something the kind of thing it is and makes it different from any other” (Wesbster, from where we learn the rarely used synonym haecceity).
iiThe essence of you, dear reader, is that you are a man (male or female), which is to say, a rational creature (in Aristotle’s sense). But you are also more than just your essence. Some of you are tall, others are not as blessed. Some have hair on head and some wear hats. That is, as Aquinas says, you are composite, made of more than one thing. But we already know God is not composite, thus He must be His own essence.
iiiHaving or not having hair, or having or not having facial freckles, is an accident. With our without, the essence behind them is still man or woman. The Aristotle reference has him saying (what is obvious) that “we must argue from a definition, viz. by assuming what falsity or truth means.” Else we go nowhere. And that “the essence of a thing is that which is expressed by its definition” (CE, above).
ivWhiteness does not go walking about on its hind legs, i.e. it is not individualized in itself. But my white hat carries on being white because the hat carries on (continues to exist).
vPerhaps it’s obvious, but that humanity exists as an essence, and that Socrates or you is not that essence, but merely examples of it, is one point. How we know it is another, as it always is. Why mention it? Well, Star Trek fans, since our essence is being a rational animal, that essence might come in other accidental packages. See this essay by Fr Schall. Or work by David Oderburg (where’s the link?).
viThis obviously follows from the premises. But on that subject, more next week, when we learn that God’s existence is His essence.
viiBack to the Unmoved Mover, the Unchanging Changer! Chapter 13, that is. See the links from last week’s review.
viiiThis follows simply from above; i.e. don’e forget the second premise “if a thing contained nothing besides its own essence, all that a thing is would be its essence.” And do meditate on the difference between potentiality and actuality. So much flows from this distinction that it isn’t funny (as my old grandma used to say about a related topic).
 Ch. xviii.
 4 Metaph. viii. 4.
 Ch. xxiii.
 Ch. xvii.
 Ch. xiii.
 Ch. xvi.