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Actus Essendi is a Latin expression coined by Saint Thomas Aquinas (1225–1274). Translated as 'act of being', the expression actus essendi is a metaphysical principle outlined by Aquinas in his Christian reinterpretation of Aristotle. It relates to the essence of a thing, and how we as humans perceive this essence. He thought one could not perceive the essence of God as sense data but rather could only be understood in terms of His act of being, that is, His effects in the real world.
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 thing to be what it is, the other is the actus essendi which gives the thing and its "essence" actual existence.
The observation that individual things display instantiations of a particular "essence" led Aquinas to postulate that what gives actual existence to a thing and to its "essence" – the actus essendi – is unique, in the sense that the perfection of actus essendi cannot be said to be common in the way an "essence" is said to be common.
Things instantiating the essence of horseness (being like a horse), for example, are said to be similar because of their horseness. The essence of horseness is what makes horses the same under a common category.
However, things instantiating the perfection of actus essendi are said to be different on account of their actus essendi. The possession of actus essendi is what makes things unique and distinct from all other things.
Thus, in what actually exists as a subsisting extramental thing, there is an "essence" which makes the thing what it is (a horse, for example), and the actus essendi which makes the thing a real, individual, existing thing.
Aristotle didn't have the notion of actus essendi. 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 actus essendi.
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". The metaphysics of Aristotle did not reach that far.
Accordingly, Pope John Paul II stresses in his teachings that the philosophy of Aquinas is the philosophy of actus essendi, "whose transcendental value paves the most direct way to rise to the knowledge of subsisting Being and pure Act, namely to God." Aquinas defined God as the "Ipse Actus Essendi subsistens", the 'subsisting act of being'.
- Thomas Aquinas, Quaestiones Disputatae De Potentia, question 7, article 2, ad 9. See also Summa Theologiae, part I, question 4, article 1, ad 3; and Summa Contra Gentiles, book II, chapter 54, no. 5.
- Thomas Aquinas, Summa Contra Gentiles, book III, chapter 66, no. 4. See also Summa Theologiae, part I, question 45, article 5, corpus.
- Pope John Paul II, "The Angelicum Address", speech delivered at the Pontifical University of Saint Thomas Aquinas Angelicum in Rome on 17 November 1979. The original, in Italian, was published in Acta Apostolicae Sedis 71 (1979): 1472–1483. English translations are available in L'Osservatore Romano English Weekly Edition (17 December 1979): 6–8; and in Angelicum 57 (1980): 133–146.
- Cornelio Fabro, "Participation", New Catholic Encyclopedia, 2nd ed. (Detroit: Gale, 2003) 10:905–910.
- Pope John Paul II, Encyclical Letter " ", 14 September 1998, Acta Apostolicae Sedis 91 (1999): 5–88.
- Natale Colafati, L'Actus Essendi in San Tommaso D'Aquino (Messina, Italy: Rubbettino Editore, 1992).
- Pope John Paul II, Apostolic Letter " ", 28 January 1999.
- Pier P. Ruffinengo, "L'ipsum esse non e ancora l'actus essendi di San Tommaso", Aquinas: Rivista Internazionale di Filosofia 38 (1995): 631–635.
- Actus Essendi and the Habit of the First Principle in Thomas Aquinas (New York: Einsiedler Press, 2019).