ONTOLOGICAL ARGUMENT. - In the history of philosophy, the o. a. is that form of demonstration which seeks to deduce the existence of God from His own concept. It is also called a simultaneo insofar as the middle term is not, ontologically, either anterior or posterior to that which one intends to demonstrate. It is therefore an analytic demonstration founded on the principle of identity, which modern logicians, however, do not distinguish from the a priori demonstration (v.).
Historically the o. a. has been presented in various forms. Its principal asserter was St. Anselm. His exposition is founded on the concept of God as "that than which nothing greater can be thought" (id quo maius cogitari nequit): such a concept is therefore inclusive of existence (Proslogion, ch. 2, 4). Against this argument Gaunilo (v.) already argued in the Liber pro insipiente (ch. 6), bringing forward the example of the fortunate islands conceived as perfect islands, which nevertheless do not exist. St. Anselm replied in the Liber apologeticus (cf. ed. Koyré, Paris 1930), showing how the concept of a "perfect island" is absurd, and therefore in no way comparable with the notion of God. According to Gilson (L'Esprit de la philosophie médiévale, I, Paris 1932, p. 63), St. Anselm's error was that he did not realize that the necessity of affirming God is only a point of support for inducing this existence. It would not be so much a proof as an initial datum of proof. Thus St. Bonaventure emphasized how the necessity of God's being quoad se is the only conceivable sufficient reason for His necessity of existing quoad nos. If one contemplates the unity of the divine essence, ipsum esse, one will see that being in itself is so certain that it cannot be thought as not existing "et videat ipsum esse adeo in se certissimum quod non potest cogitari non esse" (Itinerarium mentis in Deum, ch. 5, n. 3). Similarly for Duns Scotus the object of the intellect is being: the intellect affirms of being infinity and existence (cf. Opus Oxoniense, I, I, dist. 2, q. 1, a. 2, sect. 2, art. 2, n. 2, I, Quaracchi 1912, p. 201 ff.).
In modern philosophy Descartes (Medit., III) attempts to prove the existence of God as the necessary cause of His idea present in our mind. Moreover (ibid., V) he takes up more properly the Anselmian argument, while maintaining that he does not understand it in St. Anselm's way, insofar as he claims to start from a real essence or nature, thus avoiding the passage from the logical order to the ontological one. The Cartesian formulation is expressed thus: "To say that some attribute is contained in the nature or concept of a thing is the same as saying that this attribute is true of this thing... Now it is certain that necessary existence is contained in the nature or concept of God. Therefore it is true that necessary existence is in God, that is, that God exists" (ibid., reply to the fifth objection, §§ 56-57). In fact, the defense Descartes makes of the argument is bound to his point of view according to which being is reduced to the idea, thus denouncing the very defect of the o. a. Leibniz in the Monadology (n. 45) intends to complete the formulation of the argument given by Descartes. One must first demonstrate that God is possible, and subsequently it will follow that if He is possible He exists necessarily, insofar as in God possibility is inclusive of existence (Nouveaux Essais, I, IV, ch. 10, n. 7; cf. Letter to Arnauld of 14 July 1686, ed. Gerhardt, II, p. 63).
The o. a. was refuted in particular by St. Thomas and by Kant. St. Thomas (I Sent. dist. 3, q. 1, art. 2; De Verit. q. 10, a. 12, ad 2; C. Gent., I, 10; Sum. Theol., 1ª, q. 2, a. 1) discerns in the o. a. (in its Anselmian form) an unwarranted passage from the logical order to the real ontological order. Once the idea of a perfect being is posited, such an idea includes existence as its element, but it does not at all prove that there is actually a reality corresponding to the idea. St. Thomas insists on the distinction between what
God is quoad se and quoad nos. In other words, man does not have an a priori knowledge of God's nature that would give him, together with His essence, His existence: which must therefore be demonstrated a posteriori. Kant points out that the judgment "God exists" cannot be held to be self-evident or a priori, since being is not a real predicate, that is, the concept of something that can be added to the concept of a thing. If one takes the subject (God) with all His predicates and affirms "God exists," one does not add a new predicate to the concept of God, but merely posits the subject in itself with all its predicates. The real, therefore, Kant adds, contains nothing more than the possible. A hundred real thalers contain nothing more than a hundred possible thalers (Critique of Pure Reason, Transcendental Dialectic, ch. III, 4th section).
Despite the twofold critique, Thomist and Kantian, the o. a., as in the past, so also at present continues to have supporters. It is connected with the question of the presence and meaning of the Absolute (v.) in the human mind, so that the discussion concerning its value interferes with the more universal problem of the ultimate meaning of truth (v.) and of intellectual knowledge (v.).