Three elements of the soul control action and the discovery of truth. These are:
1. Sense perception or sensation.
2. Intelligence or reason.
3. Desire or appetite.
Sense perception, which is shared by the lower animals, has no part in initiating action and is not relevant to our discussion. Intelligence and choice (which pertains to items desired), on the other hand, together control all acts.
Moral virtue is a disposition in regard to choice, and choice, as has already been shown, is deliberate desire (i.e., choices involve desire in determining the end and intelligence in the form of calculative reason or deliberation in discovering the means to that end). While the object of reasoning in the scientific faculty is absolute truth, the object of the calculative faculty is truth in harmony with right desire (i.e., the means to satisfying right desire).
Since desire motivates action and reason determines the right rule for virtuous action, choice is based on a combination of reason and desire. Reason is the most important of the two factors, but both are essential. To choose correctly, one must determine the right rule, which means that choice is dependent on the intellectual virtues.
The starting point of any action is its source of motion, and the end of any action is its final cause. In these terms, choice is a source of motion directed towards a final cause.
