Concerning the Nature of Good, Against the Manichaeans

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.