ONTOLOGICAL ARGUMENT. — O. a. is, in the history of philosophy, that form of demonstration which intends to deduce the existence of God from his very concept. It is also called a simultaneo, since the middle term is, ontologically, neither prior nor posterior to what is to be demonstrated. It is therefore an analytical demonstration founded on the principle of identity, which modern logicians nevertheless do not distinguish from the a priori demonstration (v.).
Historically, the o. a. has been presented in various forms. Its principal proponent was s. Anselmo. His exposition is founded on the concept of God as “that than which nothing greater can be conceived” (id quo maius cogitari nequit): this concept therefore includes existence (Prologion, chap. 2, 4). Against this argument, GAUNILONE (v.) had already argued in the Liber pro insipiente (chap. 6), offering the example of the Fortunate Isles conceived as perfect islands, which nevertheless do not exist. S. Anselmo replied in the Liber apologeticus (cf. ed. Koyré, Paris 1930), showing that 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), s. Anselmo’s error was that he failed to realize that the necessity of affirming God is only a point of support for inducing this existence. It would not so much be a proof as an initial datum of proof. Thus s. Bonaventura emphasized that 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 conceived as nonexistent, “et videat ipsum esse adeo in se certissimum quod non potest cogitari non esse” (Itinerarium mentis in Deum, chap. 5, n. 3). Similarly, for Duns Scoto the object of the intellect is being: the intellect affirms of being its infinity and existence (cf. Opus Oxoniens, I, I, dist. 2, q. 1, a. 2, sect. 2, art. 2, n. 2, I, Quanschi 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 minds. Moreover (ibid., V), he takes up the Anselmian argument more properly, while maintaining that he does not understand it in the manner of s. Anselmo, since he claims to begin from a real essence or nature, thereby avoiding the passage from the logical order to the ontological order. The Cartesian formulation is expressed as follows: “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, Descartes’s defence of the argument is connected with his viewpoint, according to which being is reduced to the idea, thereby revealing the very defect of the o. a. Leibnitz, in the Monadologia (n. 45), intends to complete the formulation of the argument given by Descartes. It is first necessary to demonstrate that God is possible; subsequently it will follow that, if he is possible, he necessarily exists, since in God possibility includes existence (Nouve Essais, I, IV, chap. 10, n. 7; cf. Lettera ad Arnauld del 14 luglio 1686, ed. Gerhardt, II, p. 63).
The o. a. was refuted in particular by s. Tommaso and Kant. S. Tommaso (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 has been posited, that idea includes existence as its element, but it does not at all prove that there is in fact a reality corresponding to the idea. S. Tommaso 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; the latter must therefore be demonstrated a posteriori. Kant observes that the judgment “God exists” cannot be regarded as 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 in itself together with all its predicates. The real, Kant adds, therefore contains nothing more than the possible. One hundred real thalers contain nothing more than one hundred possible thalers (Crit. d. rag. pura., Transcendental Dialectic, chap. III, 4th section).
Despite the twofold, Thomistic and Kantian, criticism, the o. a., as in the past, so also at present, continues to have supporters. It is connected with the question of the Absolute (v.) in the human mind; hence the discussion of its value intersects with the more universal problem of the ultimate significance of truth (v.) knowledge (v.).