Chapter 10 — That God is his own essence

The further conclusion follows that God is his own essence. The essence of anything is that which its definition signifies. This is identical with the thing of which it is the definition, except accidentally, namely, insofar as something is added to the thing defined over and above its definition. Thus whiteness is added to man, over and above the fact that he is a rational and mortal animal. Hence ‘rational and mortal animal’ is the same as man, but not the same as a white man, insofar as he is white. In any being, therefore, in which there are not found two factors, of which one is essential and the other accidental, its essence must be altogether identical with it. In God, however, since he is simple (as has been shown), there are not found two factors whereof one is essential and the other accidental. Therefore, his essence must be absolutely the same as he himself.

Moreover, whenever an essence is not absolutely identical with the thing of which it is the essence, something is found in that thing by way of potency and something else by way of act. For an essence is formally related to the thing of which it is the essence as humanity is related to a man. In God, however, one does not find potency and act: he is pure act. Therefore, he is his essence.