Summa contra Gentiles1.21 Chapter XXI. That God is his own essence

From what has been laid down we are able to conclude that God is His own essence, quiddity or nature.

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 that in God there is no composition. Therefore God is His own essence.

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.

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. 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. 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.

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. 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.