is a principal virtue.

Obj. 2: Further, the principal virtues are, in a way, moral virtues.
Now we are directed to moral works both by the practical reason, and
by a right appetite, as stated in _Ethic._ vi, 2. Therefore there are
only two cardinal virtues.

Obj. 3: Further, even among the other virtues one ranks higher than
another. But in order that a virtue be principal, it needs not to
rank above all the others, but above some. Therefore it seems that
there are many more principal virtues.

_On the contrary,_ Gregory says (Moral. ii): "The entire structure of
good works is built on four virtues."

_I answer that,_ Things may be numbered either in respect of their
formal principles, or according to the subjects in which they are:
and either way we find that there are four cardinal virtues.

For the formal principle of the virtue of which we speak now is good
as defined by reason; which good is considered in two ways. First, as
existing in the very act of reason: and thus we have one principal
virtue, called "Prudence." Secondly, according as the reason puts its
order into something else; either into operations, and then we have
"Justice"; or into passions, and then we need two virtues. For the
need of putting the order of reason into the passions is due to their
thwarting reason: and this occurs in two ways. First, by the passions
inciting to something against reason, and then the passions need a
curb, which we call "Temperance." Secondly, by the passions
withdrawing us from following the dictate of reason, e.g. through
fear of danger or toil: and then man needs to be strengthened for
that which reason dictates, lest he turn back; and to this end there
is "Fortitude."

In like manner, we find the same number if we consider the subjects
of virtue. For there are four subjects of the virtue we speak of now:
viz. the power which is rational in its essence, and this is
perfected by "Prudence"; and that which is rational by participation,
and is threefold, the will, subject of "Justice," the concupiscible
faculty, subject of "Temperance," and the irascible faculty, subject
of "Fortitude."

Reply Obj. 1: Prudence is the principal of all the virtues simply.
The others are principal, each in its own genus.

Reply Obj. 2: That part of the soul which is rational by
participation is threefold, as stated above.

Reply Obj. 3: All the other virtues among which one ranks before
another, are reducible to the above four