[110] [cap.26] But we, even after we have been instructed, make no progress in learning; but some persons, having a self-taught natural instinct, purify what is good from the evils which surround it, as Jacob did, he who was surnamed the practiser of virtue; for he "peeled the rods, leaving on the white bark, having stripped off all the green;" [43] in order that the dark and dusky vanity in the middle being taken away in every case, a white appearance might be displayed, which should be produced so as to be akin to it, not by diversified art but by nature;