What is a Duty of Virtue?

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