It is one of the curiosities of history that a theology so "intel lectualist" should have profoundly influenced the thought of Christianity which is in spirit widely different from that of Aris totle. Yet the Aristotelian logic and metaphysics formed the basis of the great constructive systems of the middle ages, the scholastic theology, of which the greatest representative is St. Thomas Aquinas (1227-1274). The cosmological argument forms the basis of St. Thomas's rational theology. It was a fundamental conviction with him that the preambula fidei, the foundation truths of religion, were demonstrable by the human reason with out the aid of Revelation. In his greatest, though unfinished work, the Summa Theologica (Pt. I., Quaest. ii., art. 3), he gives five proofs of the existence of God, of which four are versions of the cosmological argument. (I ) The argument from motion: "Any thing which is moved is moved by some other thing . . .
one thing moves another in so far as the former is in actuality, for to move is nothing else than to draw anything from poten tiality to actuality. But nothing can be brought from potentiality into actuality except by means of something which is already in actuality. . . . It is impossible that in the same respect and the same manner anything should be both moving and unmoved, or be self-moved." We cannot go on to infinity in the series of "movers" which are themselves moved, for in that case there would be no first source of movement and consequently no move ment at all. We must then conclude that there is a first source of movement which is moved by nothing else—i.e., God. (2) The argument from Efficient Causes: Experience shows that there is an order of efficient causes. Nothing can be the cause of itself, for that would imply that it was prior to itself. We cannot rest content with an indefinite series of causes and effects, because if there is no First Cause there can be no last effect. Hence we con clude that there is a First and Uncaused Cause—i.e., God. (3) Argument from possible and necessary existence: Some existences are possible and not necessary, i.e., they may exist or not exist, being generated and corrupted. But all existence cannot be of this nature, for unless there were necessary existence there would be no ground for possible existence. If there are necessary existences there must be an existence which is necessary in itself and does not derive the necessity of its existence from some other necessary existence. An indefinite regress is as impossible here as in the case of efficient causes. There must therefore be Something which is necessary per se—i.e., God. (4) Argument from degree of quality or value: We find things more or less "good," "true" and "excellent." "More" or "less" is predicated according to degree of approach to a "greatest." There is therefore something which is most true, good and excellent—i.e., God.
It should be observed that the first two forms of Aquinas' cosmological argument lead to the conception of a purely Trans cendent Deity while the latter two suggest immanence.
The cosmological argument, very much in the form which was given to it by Aristotle and Aquinas, appears as a fundamental element in many philosophies. Mention must be made of Leibniz
who supplemented it by laying down a new law of thought—the law of "sufficient reason"—according to which "for everything there must be a sufficient reason why it is so and not otherwise," thus making it clear that, for him, the basis of the cosmological argument was not empirical observation but a rational and self evident principle—that of universal causation.
The objections to the traditional cosmological argument have been formulated by Hume and Kant. The former struck a blow at the simplest and most obvious version of the argument—that to a First Cause—by his sceptical analysis of the ideas of cause and necessary connection, though it should be noticed that he himself appears to have retained the conviction that the concep tion of a First Cause could not wholly be abandoned. In Hume's view, however, there is no universal principle of causation. The idea of necessary connection between phenomena is derived from habit breeding expectation, and the so-called "principle of causa tion" is due to nothing more than "the mind's propensity to feign," i.e., it is a convenient fiction. Obviously this view, which was but the logical conclusion of the empirical movement in English philosophy, undermines the whole of our knowledge of the natural order and physical science, but it has also a direct bearing on the cosmological argument, for if causation is a principle on which we cannot rely when dealing with phenomena, we cannot use it to take us beyond phenomena to God. Kant attempted to save our knowledge of Nature from Hume's sceptical objections. He did so in a somewhat equivocal fashion. He held that the "cate gories" which the mind employs in synthesizing perception (cause, substance, etc.) are a priori in the sense that the mind does not derive them from experience but necessarily uses them in order ing experience—in short that Nature apart from Mind has no existence, but in some sense "Mind makes Nature." Kant is emphatic, however, in his limitation of this principle. The cate gories of the understanding are confined to dealing with phenom ena. The use of such a category as causation to carry us beyond phenomena to a super-phenomenal Reality is an illegitimate—a "transcendent"—use. This is the real ground of Kant's objec tion; it is based upon his rigid limitation of the understanding to phenomena. Some special criticisms are also of permanent in terest. Kant points out that the argument, in the only form which he discusses (that of efficient causation), does not, even if sound, lead to the conclusion that God exists, but only that a First Cause of some kind exists, and in order to attain the conception of God we need another argument—the Ontological. With reference to the alleged impossibility of conceiving an infinite series of causes, Kant remarks that the inconceivability attaches also to the idea of an uncaused cause, and there is therefore no reason why the mind should embrace one alternative rather than the other.