The various other proofs of the existence of God given in the Middle Ages pursue two methods, one a priori, the other a posteriori. That is, one starts in Platonic fashion with the idea of a perfect being and infers its existence from this idea; the other argues, after Aristotle, from the evidences of order and perfection in the world to the idea of a perfect being who is the author of them. Anselm is an early and promi nent representative of both this and the a priori or ontological argument, which assumes that God is a being of such a nature that it is im possible to conceive any greater. The defect of this argument, as Gaunilo pointed out, consists in arguing from existence in thought to existence in fact. From the former, of course, we can logically infer nothing but an ideal thought existence.
Other theistic proofs during the Middle Ages were concerned with the course of nature and history. Thus Duns Scotus declared that the impossibility of conceiving an infinite chain of natural causes necessarily carried the mind to the idea of a great First Cause adequate to the production and preservation of the world. Aqui nas also (Surma I., qu. 2) reaches the same conclusion, a contingentia mundi, reproducing Aristotle's proofs almost word for word. The contemplation of final causes, though not exten sively meditated upon, led to very similar logi cal results from the cosmological point of view: for medieval thinkers were fond of dwelling on the fact of the imperfection of the physical and of inferring therefrom the existence of a perfect being in whose spiritual essence the soul could find the ground of the Christian faith. Thus we find that theism received the stamp of Chris tian ideas; that the greatest minds in the medice val period gave it their support because they found in it the ground of the harmony of specu lative reason and piety.
In modern times philosophical meanings have largely supplanted the theological. Thus Des cartes, the most important modern thinker on this subject, developed his theism only after sweeping aside all presuppositions derived from a supernatural source or from the symbols of the Church. Starting with the bare fact of thought (cogito, ergo sum), he argues that there must be an adequate cause for the thought of God in the mind. By this thought Descartes says that he means "a substance infinite, eternal, immutable, independent, all-knowing, all-power ful; by which I myself and every other thing have been produced." (Med. iii.) Now this thought cannot be a mere negation; for it has reality. Nor can it have arisen by adding many ideas together; for it is simple. Could it have arisen as a result of my growing intelligence? No; because the idea does not admit of growth: God is infinite always and does not admit of more or less. Hence the idea must have a divine origin. A second proof may be stated thus. We need this idea in order to explain to us the immediate existence and continuance of the universe. God is not only needed as a Creator, or logical Prins of the universe. but much more as a Preserver. A merely primary creative cause might be lost in the infinite complex of secondary causes, as in pantheism or deism. but the immediate dependence and continuance of the universe involves the idea of a preserving and sustaining cause, not only that of an orig inating one. A true cause must, therefore, be
adequate to the task of continuing the universe from moment to moment. The existence 9ZOIV of a universe involves the self-existence of its absolute Cause. As a third proof Descartes re vived Anselm's so-called ontological argument, i.e. the argument from the idea of a most perfect being to its existence. Reality, in his view, is as much a part of the idea of a perfect being, as the angles of a triangle are of the essence of our idea of a triangle, or the perfume of a rose of our idea of a rose.
Kant, in the third part of his Dialectic, criticises this argument of Descartes. He does not, indeed, deny the fact that we have the idea of a perfect being, but be doubts if front the idea we have any right to infer its real existence, since it is possible for us to have an idea (for ex ample, of a centaur) that does not correspond to any object. And inasmuch as all possible proofs of the existence of God are reducible to this one, the ontological, Kant considers that all the argu ments of Descartes and of the mediaeval thinkers failed to establish their point. Accordingly he proposed a new and infallible proof, the so called practical or moral arguments for the existence of God, which is as follows. We have the notion of a moral law; conscience responds to the categorical moral imperative. Universal experience proves that happiness and virtue in volve each other and cannot be separated. Obe dience to these moral conditions is the law of life, of conduct, and character; for the wicked are never, in the long run, happy. Now, said Kant, for this conviction there is needed a cause, supreme and infinite: a cause capable of clinch ing this relation between happiness and virtue to all eternity; a cause which will secure the triumph of justice as against the moral in equalities of the present life. This moral cause we call, by faith, God. Thus the only ground for insisting on `proofs' of the existence of this cause is for Kant the subjective need of a cause of the moral law within our souls, and further than this Kant held it was impossible for the human mind to go.
Beget disagreed with Kant's conclusions, and sought to revive the ontological argument of Ansehu and Descartes, in a new form, contending that the idea of a perfect being was an expres sion of the nature of all thought and all reality. He refused to believe that the traditional sepa ration of thought and thing, upon which Kant depended, represented any real or valuable dis tinction. For him thought and reality are the same. We cannot ever get 'beyond' thought. To put a barrier, as Kant did, between thought and thing is, for Hegel, to cease to think. All recent reflection on the theistic problem rests on this idealistic presupposition of Hegel. He may, therefore, be regarded as furnishing present day theism with its most valuable logical founda tion.
Consult: Harris, The Philosophical Basis of Theism. (New York. 1883) ; Flint, Theism (Ediu burgh, 1887) ; Ladd, The Doctrine of Sacred Scripture (New York, 1884) ; Boyne, Theism (ib., 1903) : Fisher, Grounds of Theistie and Christian Belief (ib., 1883; new ed., ib., 1903).