(from Greek ὄν and λόγος, i.e. the science of being) is, strictly speaking a synonyme of metaphysics (q.v.), but neither the one name nor the other was used by Aristotle. He called the science now designated by them philosophia prima, and defined it as ἐπιστήμη τοῦ ὄντος ῃ ὄντος — Scientia. Entis quatenus Entis — that is, the science of the essence of things; the science of the attributes and conditions of being in general, not of being in any given circumstances, not as physical or mathematical, but as being.
The science of ontology is regarded as comprehending investigations of every real existence, either beyond the sphere of the present world, or in any other way incapable of being the direct object of consciousness, or which can be deduced immediately from the possession of certain feelings or principles and faculties of the human soul (comp. Butler; Lectures on Ancient Philosophy, vol. 2). Watts thus defines it: "Ontology is a discourse of being in general, and the various or most universal modes or affections, as well as the several kinds or divisions of it. The word being here includes not only whatsoever actually is, but whatsoever can be" (On Ontology, ch. ii). The name ontology seems to have been first made current in philosophy by Wolf. He divided metaphysics into four parts: Ontology, psychology, rational cosmology, and theology. It was chiefly occupied with abstract inquiries into possibility; necessity, and contingency, substance, accident, cause, etc., without reference to the laws of our intellect by which we are constrained to believe in them. Kant denied that we have any knowledge of substance or cause as really existing. But there is a science of principles and causes, of the principles of being and knowing. In this view of it, ontology corresponds to metaphysics. Ontology may be treated of in two different methods, according as its exponent is a believer in τὸ ὄν or in τὰ ὄντα , in one or in many fundamental principles of things. In the former, all objects whatever are regarded as phenomenal modifications of one and the same substance, or as self-determined effects of one and the same cause. The necessary result of this method is to reduce all metaphysical philosophy to a rational theology. the one substance or cause being identified with the Absolute or the Deity. According to the latter method, which professes to treat of different classes of beings independently, metaphysics will contain three co-ordinate branches of inquiry — rational cosmology, rational psychology, and rational theology. The first aims at a knowledge of the real essence, as distinguished from the phenomena of the material world; the second discusses the nature and origin, as distinguished from the faculties and affections; the third aspires to comprehend God himself, as cognizable a priori in his essential nature, apart from the indirect and relative indications furnished by his works, as in Natural Theology (q.v.), or by his Word, as in Revealed Religion (q.v.). These three objects of metaphysical inquiry God, the world, the mind-correspond to Kant's three ideas of the Pure Reason; and the object of his Kritik is to show that, in relation to all these, the attainment of a system of speculative philosophy is impossible (Mansel, Prolegom. Log. p.:272).
In theology the ontological argument has been freely employed, especially in the Middle Ages, regarding the Being of God. St. Augustine used it, so did Boethius; but it was left for Anselm to develop it fully. They all three inferred the existence of God from the existence of general ideas. Thus Augustine taught (De Lib. Arbitr. lib. ii, c. 3-15) that there are general ideas which have for every one the same objective validity, and are not (like the perceptions of sense) different and conditioned by the subjective apprehension. Among these are the mathematical truths, as 3+7=10; here, too, belongs the higher metaphysical truth — truth in itself, i.e. wisdom (veritas, sapientla). The absolute truth, however, which is necessarily demanded by the human mind, is God himself. Augustine asserts that man is composed of existence, life, and thinking, and shows that the last is the most excellent; hence he infers that that by which thinking is regulated, and which, therefore, must be superior to thinking itself, is the summum bonum. He finds this summum bonum in those general laws which every thinking person must acknowledge, and according to which he must form an Opinion respecting thinking itself. The sum total of these laws or rules is called truth or wisdom (veritas, sapientia). The absolute is, therefore, equal to truth itself. God is truth. (Comp. Ritter, Christl. Philippians 1:407- 411.) Boethius expresses himself still more definitely (De Consol. Philippians v. Prosa 10): he shows that empirical observation and the perception of the imperfect lead necessarily to the idea of perfection and its reality in God. (Comp. Schleiermacher, Geschichte der Philosophie, p. 166.) Of Anselm's argument we can here give only the heads; the thread of reasoning must be seen from the connection:
"Monol. I. Cum tam inuumtlerabilia bona sint; quorum tam.mnltam diversitatem et sensibus corporeis experimur et ratione mentis discernimus, estne credendum esse unum aliquid, per quod unum sunt bona, qusecunque bona saunt, ant sunt bona alia per aliud? III. Demique non solum omnia bona per idem aliquid sunt boina et omnia magna per idem aliquid sunt magna, sed quicquid est, per unumu aliquid videtur esse... Quoniam ergo cuncta quse sunt, Sunt per ipsum unum; procul dubio et ipsum sunum est per se ipsum. Qusecunque igitur alia sunt, sunt per alind, et ipsum solum per se ipsum. Ac quicquid est per aliud, ninus est quam illud, per quod cuncta sunt alia et quod solum est per se: quareillud, quod est per se, maxime omnium est. Est igitur unum aliquid, qnod solnm maxime.et summe omnium est; quod autem maxime omnium est et per quod est quicquid est bonilm vel magiram, et omnino qnicquid est aliquidc est, id necesse est esse summe bonum et summe magnum et summum omnium. que sunlt. Quare est aliquid, quod sive essentia, sive substantia, sive natura dicatur, optimum et maximum est et summum omnium quae sunt."
The mode of argument which is found in Proslog. c. ii is more original (he there proceeds from the reality of the idea): The fool may say in his heart there is no God (Psalms 14:1), but he thereby shows himself a fool, because he asserts something which is contradictory in itself. He has the idea of God in him, but denies its reality. But if God is given in idea, he must also exist in reality. Otherwise the real God, whose existence is conceivable, would be superior to the one who exists only in imagination, and consequently would be superior to the highest conceivable object, which is absurd; hence it follows that that beyond which nothing can be conceived to exist really exists (thus idea and reality coincide). If, therefore, the fool says, There is no God, he says it indeed, and may, perhaps, even think it. But there is a difference between thought and thought. To conceive a thing when the word is without meaning, e.g. that fire is water (a mere sound, an absurdity!), is very different from the case in which the thought corresponds with the word. It is only according to the former mode of thinking (which destroys the thought itself) that the fool can say, There is no God, but not according to the latter. See Ueberweg, Hist. of Philos.' i,. 378, 383 sq;; 2:42, 49, 56, 104 sq., 148, 177, 497 sq.; M;Cosh, Intuition of God; Farrar, Crit. Hist. of Free Thought; Morell, Hist. of Philos. 18th and 19th Cent. p. 653; Baur; Dogmengesch. vol. ii; Hagenbach, Hist. of Doctrines, 1:325 sq.; Krauth's Vining, Voccbulary of Philos. s.v.; Cocker, Christianity and Greek Philos. p. 491-494.