Actus essendi

Actus essendi is a Latin expression coined by Saint Thomas Aquinas (1225–1274). Translated as "act of being", the actus essendi is a fundamental metaphysical principle discovered by Aquinas when he was systematizing the Christian Neoplatonic interpretation of Aristotle. The metaphysical principle of actus essendi relates to the revelation of God as He Who Is (Exodus 3:14), and to how we as humans perceive God’s essence. Aquinas elaborates on the fact that God’s essence is not perceived as sense data; rather, the essence of God can only be understood partially in terms of the limited participations in God’s actus essendi, that is, in terms of what is real, in terms of God’s effects in the real world.

Aquinas saw the metaphysical principle of actus essendi as the "act of all acts, the perfection of all perfections", and "a proper effect of God".