d to be three
things existing in the soul, power, passion, and habit. Power is the
principle or matter of passion, as power to be angry, ashamed, or
confident: and passion is the actual setting in motion of that power,
being itself anger, confidence, or shame; and habit is the strong
formation of power in the unreasoning element engendered by use, being
vice if the passions are badly tutored by reason, virtue if they are
well tutored.

Sec. V. But since they do not regard every virtue as a mean, nor call it
moral, we must discuss this difference by approaching the matter more
from first principles. Some things in the world exist absolutely, as the
earth, the sky, the stars, and the sea; others have relation to us, as
good and evil, as what is desirable or to be avoided, as pleasant and
painful: and since reason has an eye to both of these classes, when it
considers the former it is scientific and contemplative, when it
considers the latter it is deliberative and practical. And prudence is
the virtue in the latter case, as knowledge in the former. And there is
this difference between prudence and knowledge, prudence consists in
applying the contemplative to the practical and emotional so as to make
reason paramount. On which account it often needs the help of fortune;
whereas knowledge needs neither the help of fortune nor deliberation to
gain its ends: for it considers only things which are always the same.
And as the geometrician does not deliberate about the triangle, as to
whether its interior angles are together equal to two right angles, for
he knows it as a fact--and deliberation only takes place in the case of
things which differ at different times, not in the case of things which
are certain and unchangeable--so the contemplative mind having its scope
in first principles, and things that are fixed, and that ever have one
nature which does not admit of change, has no need for deliberation. But
prudence, which has to enter into matters full of obscurity and
confusion, frequently has to take its chance, and to deliberate about
things which are uncertain, and, in carrying the deliberation into
practice, has to co-operate with the unreasoning element, which comes to
its help, and is involved in its decisions, for they need an impetus.
Now this impetus is given to passion by the moral character, an impetus
requiring reason to regulate it, that it may render moderate and not
excessive help, and at the seasonable time.