Actus essendi is a Latin expression coined by Saint Thomas Aquinas (1225–1274). Translated as "act of being", the Latin: actus essendi is a fundamental metaphysical principle discovered by Aquinas when he was systematizing the Christian Neoplatonic interpretation of Aristotle. The metaphysical principle of Latin: 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 Latin: 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 Latin: actus essendi as the "act of all acts, the perfection of all perfections",[1] and "a proper effect of God".[2]