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 may be called duties of ............