§ 87. _Theology_, as a branch of philosophy, deals with _the proof and the nature of God_. Since "God" is not primarily a theoretical conception, the proof of God is not properly a philosophical problem. Historically, this task has been assumed as a legacy from Christian apologetics; and it has involved, at any rate so far as European philosophy is concerned, the definition of ultimate being in such spiritual terms as make possible the relation with man postulated in Christianity. For this it has been regarded as sufficient to ascribe to the world an underlying unity capable of bearing the predicates of perfection, omnipotence, and omniscience. Each proof of God has defined him pre-eminently in terms of some one of these his attributes. [Sidenote: The Ontological Proof of God.] § 88. The _ontological_ proof of God held the foremost place in philosophy's contribution to Christianity up to the eighteenth century. This proof _infers the existence from the ideal_ of God, and so approaches the nature of God through the attribute of perfection. It owes the form in which it was accepted in the Middle Ages and Renaissance to St. Anselm, Archbishop of Canterbury at the close of the eleventh century. He argued from the idea of a most perfect being to its existence, on the ground that non-existence, or existence only in idea, would contradict its perfection. It is evident that the force of this argument depends upon the necessity of the idea of God. The argument was accepted in Scholastic Philosophy[201:15] largely because of the virtual acceptance of this necessity. Mediæval thought was under the dominance of the philosophical ideas of Plato and Aristotle, and through them rationalism had come to be the unquestioned starting-point for all thought. For Plato reality and rationality meant one and the same thing, so that the ultimate reality was the highest principle of rationality, which he conceived to be the idea of the good. In the case of Aristotle the ideal of rationality was conceived to determine the course of the cosmical evolution as its immanent final cause. But in itself it was beyond the world, or transcendent. For Plato perfection itself is reality, whereas for Aristotle perfection determines the hierarchical order of natural substances. The latter theory, more suitable to the uses of Christianity, because it distinguished between God and the world, was incorporated into the great school systems. But both theories contain the essence of the ontological proof of God. In thought one seeks the perfect truth, and posits it as at once the culmination of insight and the meaning of life. The ideal of God is therefore a necessary idea, because implied in all the effort of thought as the object capable of finally satisfying it. St. Anselm adds little to the force of this argument, and does much to obscure its real significance.