De Plantatione

According, then, to the former statement one may understand it in this manner: the three years being taken for time which is divided into three portions; for it is the nature of time to be divided into the past, the present, and the future; therefore the fruit of education will exist, and will endure, and will last unimpaired through all the divisions of time, a statement equivalent to—it will never receive any corruption, for the nature of good is imperishable. But the fruit which is not purified shall not be eaten; inasmuch as virtuous reasons, duly purified and rendered sound, nourish the soul, and give vigour to the mind; but the opposite kinds are not nutritious, but bring disease and destruction on the soul.

According to the other meaning, as in the disputes of dialecticians, the word "undemonstrated" is used in a double sense, either of a proposition which it is hard to demonstrate by reason of its difficulty, or of one which is intrinsically so plain as to require no demonstration, and the truth of which is established not by the testimony of any one else, but by its own internal evidence. So also fruit may be understood as not being purified, either when it is so impure as to be difficult to purify, or when it requires no purification, but is bright, and clear, and pure of itself.