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 virtue (officia honestatis), just because they are subject only to free self-constraint, not to the constraint of other men, and determine the end which is also a duty. Virtue, being a coincidence of the rational will, with every duty firmly settled in the character, is, like everything formal, only one and the same. But, as regards the end of actions, which is also duty, that is, as regards the matter which one ought to make an end, there may be several virtues; and as the obligation to its maxim is called a duty of virtue, it follows that there are also several duties of virtue. {INTRODUCTION ^paragraph 90} The supreme principle of ethics (the doctrine of virtue) is: "Act on a maxim, the ends of which are such as it might be a universal law for everyone to have." On this principle a man is an end to himself as well as others, and it is not enough that he is not permitted to use either himself or others merely as means (which would imply that be might be indifferent to them), but it is in itself a duty of every man to make mankind in general his end.