Thomas Aquinas, Compendium of Theology

This state enjoyed by man depended on the submission of the human will to God. That man might be accustomed from the very beginning to follow God's will, God laid certain precepts on him. Man was permitted to eat of all the trees in Paradise, with one exception: he was forbidden under pain of death to eat of the tree of knowledge of good and evil. Eating of the fruit of this tree was prohibited, not because it was evil in itself, but that at least in this slight matter man might have some precept to observe for the sole reason that it was so commanded by God. Hence eating of the fruit of this tree was evil because it was forbidden. The tree was called the tree of knowledge of good and evil, not because it had the power to cause knowledge, but because of the sequel: by eating of it man learned by experience the difference between the good of obedience and the evil of disobedience.