Concerning the Nature of Good, Against the Manichæans 35
35 The Tree Was Forbidden to Adam Not Because It Was Evil, But Because It Was Good for Man to Be Subject to God PL 42, 562
For besides, He had made the prohibition, in order to show that the nature of the rational soul ought not to be in its own power, but in subjection to God, and that it guards the order of its salvation through obedience, corrupting it through disobedience. Hence also He called the tree, the touching of which He forbade, the tree "of the knowledge of good and evil;" because when man should have touched it in the face of the prohibition, he would experience the penalty of sin, and so would know the difference between the good of obedience, and the evil of disobedience.
Arbor Adamo vetita, non quia mala, sed quia homini bonum ut subditus sit Deo. Ad hoc enim et prohibuerat, ut ostenderet animae rationalis naturam, non in sua potestate, sed Deo subditam esse debere, et ordinem suae salutis per obedientiam custodire, per inobedientiam corrumpere. Hinc et arborem quam tangi vetuit, sic appellavit, dignoscentiae boni et mali : quia cum eam contra vetitum tetigisset, experiretur poenam peccati, et eo modo dignosceret quid interesset inter obedientiae bonum et inobedientiae malum.