Prophets of the Old Testament, and, second, but in fact primarily, in the life and words of Jesus Christ, "the Word." Of the antiquity and reliability of this first source--the Prophets--they were never tired of talking, and they were so confident of the necessity of resorting to it that they developed their famous theory of the indebtedness of Plato and Aristotle to those Hebrew seers for their theology. "From every point of view, therefore," concludes St. Justin Martyr, "it must be seen that in no other way than only from the prophets, who teach us by divine inspiration, is it at all possible to learn anything concerning God and the true religion."[50] But the chief source from which the Fathers drew that certainty which they could not find in the demonstrations of philosophy was in the teaching of the Word, Jesus Christ. God, indeed, as we have seen, is not an object of science, "but the Son is wisdom and knowledge and truth, and all else that has affinity thereto. _He is also susceptible of demonstration and of description._"[51] It is in the incarnate Word of God that the patristic writers find expressed all that man is able to comprehend, and all that he needs to know in this present world, of the Divine Nature, and it is His words that confirm their confidence in that "innate opinion" of the existence of God, of the presence of which in every man they were so sure. The subject of the "demonstration" of the existence of God is spoken of at some length in several places by St. Clement of Alexandria, and with his position most of the Fathers agree in the main. He regards the subject largely from an Aristotelian point of view. All knowledge is derived from Sensation and Understanding. "Intellectual apprehension is first in the order of nature; but in our case, and in relation to ourselves, Sensation is first, and of Sensation and Understanding the essence of knowledge is formed, and evidence is common to Understanding and Sensation."[52] But "should any one say that knowledge is founded on demonstration" (which "depends on primary and better known principles,"[53] being "discourse agreeable to reason, producing belief in points disputed, from points admitted"[54]) "_by a process of reasoning_, let him hear that first principles are incapable of demonstration, for they are neither by art" (τέχνη), which is "practical solely, and not theoretical," "nor by sagacity" (φρόνησις = practical wisdom), which is "conversant about objects which are susceptible of change,"[55] but are "primary," "self-evident," and "indemonstrable."[56] Thus this "demonstration by a process of reasoning," apart from Sensation _and_ Understanding, is only "to syllogize;" "for to draw the proper conclusion from the premisses is merely to syllogize. But to have also each of the premisses true is not merely to have syllogized, but also to have demonstrated," "so that if there is demonstration at all, there is an absolute necessity that there