Moreover, there are two things which appear to fall under the subject of philosophy—knowledge and supposition; and if these are taken away, philosophy altogether falls to the ground. But the chief of the philosophers themselves have taken away both from philosophy. Socrates took away knowledge, Zeno supposition. Let us see whether they were right in doing so. Wisdom is, as Cicero defined it, 1493 the knowledge of divine and human things. Now if this definition is true, wisdom does not come within the power of man. For who of mortals can assume this to himself, to profess that he knows divine and human things? I say nothing of human affairs; for although they are connected with divine, yet, since they belong to man, let us grant that it is possible for man to know them. Certainly he cannot know divine things by himself, since he is a man; whereas he who knows them must be divine, and therefore God. But man is neither divine nor God. Man, therefore, cannot thoroughly know divine things by himself. No one, therefore, is wise but God, or certainly that man whom God has taught. But they, because they are neither gods, nor taught by God, cannot be wise, that is, acquainted with divine and human things. Knowledge, therefore, is rightly taken away by Socrates and the Academics. Supposition also does not agree with the wise man. For every p. 234 one supposes that of which he is ignorant. Now, to suppose that you know that of which you are ignorant, is rashness and folly. Supposition, therefore, was rightly taken away by Zeno. If, therefore, there is no knowledge in man, and there ought to be no supposition, philosophy is cut up by the roots.
De Offic., ii. 2.