ONTOLOGICAL ARGUMENT. — The ontological argument, in the history of philosophy, is that form of demonstration which seeks to deduce the existence of God from His very concept. It is also called simultaneous inasmuch as the middle term is, ontologically, neither prior nor posterior to that which one seeks 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 (q.v.).
Historically, the ontological argument has been presented in various forms. Its principal advocate was St. Anselm. His exposition is based 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, cap. 2, 4). Against this argument, Gaunilo (q.v.) in the Liber pro insipiente (cap. 6) argued by citing the example of the fortunate isles conceived as perfect islands, which nonetheless 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 thus in no way comparable to the notion of God. According to Gilson (L’Esprit de la philosophie médiévale, I, Paris 1932, p. 63), St. Anselm’s error was in not realizing that the necessity of affirming God is merely a starting point 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 of as non-existent: “et videat ipsum esse adeo in se certissimum quod non potest cogitari non esse” (Itinerarium mentis in Deum, cap. 5, n. 3). Similarly, for Duns Scotus, the object of the intellect is being: the intellect affirms of being its infinity and its existence (cf. Opus Oxoniense, I, I, dist. 2, q. 1, a. 2, sect. 2, art. 2, n. 2, I, Quaracchi 1912, p. 201 et seq.).
In modern philosophy, Descartes (Medit., III) attempts to prove the existence of God as the necessary cause of the idea of Him present in our mind. Moreover (ibid., V), he more properly resumes the Anselmian argument, though claiming not to understand it in St. Anselm’s manner, inasmuch as he affirms that he starts from a real essence or nature, thus avoiding the passage from the logical to the ontological order. Descartes’ formulation is thus expressed: “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, God exists” (ibid., reply to the fifth objection, §§ 56-57). In fact, Descartes’ defense of the argument is tied to his viewpoint according to which being is reduced to the idea, thereby betraying the very flaw of the ontological argument. 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, since in God possibility is inclusive of existence (Nouveaux Essais, I, IV, cap. 10, n. 7; cf. Letter to Arnauld of 14 July 1686, ed. Gerhardt, II, p. 63).
The ontological argument was refuted in particular by St. Thomas Aquinas 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., I, q. 2, a. 1) discerns in the ontological argument (in its Anselmian form) an unwarranted passage from the logical to the real ontological order. Once the idea of a perfect being is posited, such an idea includes existence as one of its elements, but it does not in any way prove that there actually exists 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 the nature of God which would give him, together with His essence, His existence; this must therefore be demonstrated a posteriori. Kant notes that the judgment “God exists” cannot be considered 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 its predicates and affirms “God exists,” one does not add a new predicate to the concept of God, but merely posits the subject itself with all its predicates. The real, Kant adds, contains nothing more than the possible. A hundred real thalers do not contain anything more than a hundred possible thalers (Critique of Pure Reason, Transcendental Dialectic, ch. III, 4th section).
Despite the double critique, both Thomist and Kantian, the ontological argument, 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 (q.v.) in the human mind, so that the discussion of its value interferes with the more universal problem of the ultimate meaning of truth (q.v.) and of intellectual knowledge (q.v.).