Augustine, The City of God, Book VIII, Chs.4,5
To Plato is given the praise of having perfected philosophy by combining both parts into one. He then divides it into three parts, — the first moral, which is chiefly occupied with action; the second natural, of which the object is contemplation; and the third rational, which discriminates between the true and the false. And though this last is necessary both to action and contemplation, it is contemplation, nevertheless, which lays peculiar claim to the office of investigating the nature of truth. Thus this tripartite division is not contrary to that which made the study of wisdom to consist in action and contemplation. Now, as to what Plato thought with respect to each of these parts, — that is, what he believed to be the end of all actions, the cause of all natures, and the light of all intelligences, — it would be a question too long to discuss, and about which we ought not to make any rash affirmation. For, as Plato liked and constantly affected the well-known method of his master Socrates, namely, that of dissimulating his knowledge or his opinions, it is not easy to discover dearly what he himself thought on various matters, any more than it is to discover what were the real opinions of Socrates. We must, nevertheless, insert into our work certain of those opinions which he expresses in his writings, whether he himself uttered them, or narrates them as expressed by others, and seems himself to approve of, — opinions sometimes favorable to the true religion, which our faith takes up and defends, and sometimes contrary to it, as, for example, in the questions concerning the existence of one God or of many, as it relates to the truly blessed life which is to be after death. For those who are praised as having most closely followed Plato, who is justly preferred to all the other philosophers of the Gentiles, and who are said to have manifested the greatest acuteness in understanding him, do perhaps entertain such an idea of God as to admit that in Him are to be found the cause of existence, the ultimate reason for the understanding, and the end in reference to which the whole life is to be regulated. Of which three things, the first is understood to pertain to the natural, the second to the rational, and the third to the moral part of philosophy. For if man has been so created as to attain, through that which is most excellent in him, to that which excels all things, — that is, to the one true and absolutely good God, without whom no nature exists, no doctrine instructs, no exercise profits, — let Him be sought in whom all things are secure to us, let Him be discovered in whom all truth becomes certain to us, let Him be loved in whom all becomes right to us.