Virtue is the strength of the man's maxim in his obedience to
duty. All strength is known only by the obstacles that it can
overcome; and in the case of virtue the obstacles are the natural
inclinations which may come into conflict with the moral purpose;
and as it is the man who himself puts these obstacles in the way of
his maxims, hence virtue is not merely a self-constraint (for that
might be an effort of one inclination to constrain another), but is
also a constraint according to a principle of inward freedom, and
therefore by the mere idea of duty, according to its formal law.

All duties involve a notion of necessitation by the law, and ethical
duties involve a necessitation for which only an internal
legislation is possible; juridical duties, on the other hand, one
for which external legislation also is possible. Both, therefore,
include the notion of constraint, either self-constraint or constraint
by others. The moral power of the former is virtue, and the action
springing from such a disposition (from reverence for the law) may
be called a virtuous action (ethical), although the law expresses a
juridical duty. For it is the doctrine of virtue that commands us to
regard the rights of men as holy.

But it does not follow that everything the doing of which is virtue,
is, properly speaking, a duty of virtue. The former may concern merely
the form of the maxims; the latter applies to the matter of them,
namely, to an end which is also conceived as duty. Now, as the ethical
obligation to ends, of which there may be many, is only indeterminate,
because it contains only a law for the maxim of actions, and the end
is the matter (object) of elective will; hence there are many
duties, differing according to the difference of lawful ends, which