Actus essendi is a Latin expression coined by Saint Thomas Aquinas (1225–1274). Translated as "act of being", the is a fundamental metaphysical principle discovered by Aquinas when he was systematizing the Christian Neoplatonic interpretation of Aristotle. The metaphysical principle of relates to the revelation of God as He Who is (Ex 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 , that is, in terms of what is real, in terms of God’s effects in the real world. Overview Aquinas saw that in any subsisting extramental (existing outside the mind) thing, one finds a couplet of metaphysical principles: one is the "essence" which makes the subsisting thing to be what it is, the other is the which gives the subsisting thing and its essence actual existence. The observation that individual subsisting things display instantiations of a particular essence led Aquinas to postulate that what gives actual existence to a subsisting thing and to its essence – the – is unique, in the sense that the perfection of cannot be said to be common in the way an essence is said to be common. Subsisting things instantiating the essence of (real horses), for example, are said to be similar because of their . The essence of is what makes horses the same under a common category. However, subsisting things instantiating the perfection of are said to be different on account of their . The possession of is what makes a subsisting thing unique and distinct from all other subsisting things. Thus, in what actually exists as a subsisting extramental thing, there is an essence which makes the subsisting thing what it is (a horse, for example), and the which makes the subsisting thing a real, individual, existing thing. Aristotle didn't have the notion of . In fact, the contribution of Aquinas to the philosophy of being is precisely that he discovered that all Aristotelian acts were in reality "potency" with respect to the . Aquinas saw the metaphysical principle of as the "act of all acts, the perfection of all perfections", and "a proper effect of God". The metaphysics of Aristotle did not reach that far. Accordingly, Pope John Paul II stresses in his teachings that the philosophy