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.