Didascalicon, Book I (chapters 2-13): the origin of the arts and the division of philosophy
Chapter VI. On the rise of theoretical, practical and mechanical knowledge
Now the end and intention of all human actions or studies that are governed by wisdom ought to look to this: that either the integrity of our nature be restored, or the necessity of the defects to which the present life is subject be tempered. I will say more plainly what I have said. There are two things in man: good and evil, nature and vice. The good, because it is nature, because it is corrupted, because it is lessened, must be restored by exercise. The evil, because it is vice, because it is corruption, because it is not nature, must be excluded. And if it cannot be utterly exterminated, it must at least be tempered by the application of a remedy. This is altogether what is to be done: that nature be restored and vice excluded. The integrity of human life, moreover, is perfected by two things, knowledge and virtue, which alone are our likeness to the higher and divine substances. For since man is not of a simple nature but is compacted of a twofold substance, according to one part of himself, which is the better (and, to say more plainly what is needed, which he himself is), he is immortal; but according to the other part, which is perishable, which alone is known to those who know how to trust nothing but the senses, he is subject to mortality and change, where it is necessary to die as often as one loses what one is. And this is the last part of things, which has a beginning and an end.