Book 6: The Intellectual Virtues.
Chapter 2: The function of the intellect, both in practice and speculation, is to attain truth.
Now, the faculties which guide us in action and in the apprehension of truth are three: sense, reason, and desire.
The first of these cannot originate action, as we see from the fact that brutes have sense but are incapable of action.
If we take the other two we find two modes of reasoning, viz. affirmation and negation [or assent and denial], and two corresponding modes of desire, viz. pursuit and avoidance [or attraction and repulsion].
Now, moral virtue is a habit or formed faculty of choice or purpose, and purpose is desire following upon deliberation.
It follows, then, that if the purpose is to be all it should be, both the calculation or reasoning must be true and the desire right, and that the very same things must be assented to by the former and pursued by the latter.
This kind of reasoning, then, and this sort of truth has to do with action.
But speculative reasoning that has to do neither with action nor production is good or bad according as it is true or false simply: for the function of the intellect is always the apprehension of truth; but the function of the practical intellect is the apprehension of truth in agreement with right desire.
Purpose, then, is the cause—not the final but the efficient cause or origin—of action, and the origin of purpose is desire and calculation of means; so that purpose necessarily implies on the one hand the faculty of reason and its exercise, and on the other hand a certain moral character or state of the desires; for right action and the contrary kind of action are alike impossible without both reasoning and moral character.
Mere reasoning, however, can never set anything going, but only reasoning about means to an end—what may be called practical reasoning (which practical reasoning also regulates production; for in making anything you always have an ulterior object in view—what you make is desired not as an end in itself, but only as a means to, or a condition of, something else; but what you do is an end in itself, for well-doing or right action is the end, and this is the object of desire).
Purpose, then, may be called either a reason that desires, or a desire that reasons; and this faculty of originating action constitutes a man.
No past event can be purposed; e.g. no one purposes to have sacked Troy; for no one deliberates about that which is past, but about that which is to come, and which is variable: but the past cannot be undone; so that Agathon is right when he says—
“This thing alone not God himself can do—
To make undone that which hath once been done.”
We have thus found that both divisions of the reason, or both the intellectual faculties, have the attainment of truth for their function; that developed state of each, then, in which it best attains truth will be its excellence or virtue.