Actus purus

In scholastic philosophy, actus purus ( literally "pure act") is the absolute perfection of God.

Created beings have potentiality that is not actuality, imperfections as well as perfection. Only God is simultaneously all that He can be, infinitely real and infinitely perfect: 'I am who I am' (Exodus 3:14). His attributes or His operations, are really identical with His essence, and His essence necessitates His existence. (Contrast this understanding with the Essence–Energies distinction in Eastern Christian, particularly Palamite, theology).

In created beings, the state of potentiality precedes that of actuality; before being realized, a perfection must be capable of realization. But, absolutely speaking, actuality precedes potentiality. For in order to change, a thing must be acted upon, or actualized; change and potentiality presuppose, therefore, a being which is in actu. This actuality, if mixed with potentiality, presupposes another actuality, and so on, until we reach the actus purus.