We must touch, upon the excellence or best condition of both of them (I).

There are three principal functions of the soul--Sensation, Reason, and Appetite or Desire.

Now, Sensation (which beasts have as well as men) is not a principle of moral action.
The Reason regards truth and falsehood only; it does not move to action, it is not an end in itself.

Appetite or Desire, which aims at an end, introduces us to moral action.

Truth and Falsehood, as regards Reason, correspond to Good and Evil as regards Appetite: Affirmation and Negation, with the first, are the analogues of Pursuit and Avoidance, with the second.