Ch. 10 "The Identity of God with His Essence" from "The Compendium of Theology" by St. Thomas Aquinas

Caput 10 Quod Deus est sua essentia

CHAPTER 10 IDENTITY OF GOD WITH HIS ESSENCE

Sequitur autem ulterius quod Deus sit sua essentia. Essentia enim uniuscuiusque rei est illud quod significat definitio eius. Hoc autem est idem cum re cuius est definitio, nisi per accidens, inquantum scilicet definito accidit aliquid quod est praeter definitionem ipsius; sicut homini accidit albedo praeter id quod est animal rationale et mortale: unde animal rationale et mortale est idem quod homo, sed non idem homini albo inquantum est album. In quocumque igitur non est invenire duo, quorum unum est per se et aliud per accidens, oportet quod essentia eius sit omnino idem cum eo. In Deo autem, cum sit simplex, ut ostensum est, non est invenire duo quorum unum sit per se, et aliud per accidens. Oportet igitur quod essentia eius sit omnino idem quod ipse.

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, unless per accidens 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 whiteness, so far as it is white, is not the same as man. In any being, therefore, in which there are not found two factors whereof one is per se and the other per accidens, 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 per se and the other per accidens. Therefore His essence must be absolutely the same as He Himself.

Item. In quocumque essentia non est omnino idem cum re cuius est essentia, est invenire aliquid per modum potentiae, et aliquid per modum actus, nam essentia formaliter se habet ad rem cuius est essentia, sicut humanitas ad hominem: in Deo autem non est invenire potentiam et actum, sed est actus purus; est igitur ipse sua essentia.

Moreover, whenever an essence is not absolutely identical with the thing of which it is the essence, something is discerned in that thing that has the function of potency, and something else that has the function of act. For an essence is formally related to the thing of which it is the essence as humanity is related to man. In God. however, no potency and act can be discerned: He is pure act. Accordingly He is His essence.