The notion of duty is in itself already the notion of a constraint of the free elective will by the law; whether this constraint be an external one or be self-constraint. The moral imperative, by its categorical (the unconditional ought) announces this constraint, which therefore does not apply to all rational beings (for there may also be holy beings), but applies to men as rational physical beings who are unholy enough to be seduced by pleasure to the transgression of the moral law, although they themselves recognize its authority; and when they do obey it, to obey it unwillingly (with resistance of their inclination); and it is in this that the constraint properly consists.[*] Now, as man is a free (moral) being, the notion of duty can contain only self-constraint (by the idea of the law itself), when we look to the internal determination of the will (the spring), for thus only is it possible to combine that constraint (even if it were external) with the freedom of the elective will. The notion of duty then must be an ethical one.
[*]Man, however, as at the same time a moral being, when he considers himself objectively, which he is qualified to do by his pure practical reason, (i.e., according to humanity in his own person). finds himself holy enough to transgress the law only unwillingly; for there is no man so depraved who in this transgression would not feel a resistance and an abhorrence of himself, so that he must put a force on himself. It is impossible to explain the phenomenon that at this parting of the ways (where the beautiful fable places Hercules between virtue and sensuality) man shows more propensity to obey inclination than the law. For, we can only explain what happens by tracing it to a cause according to physical laws; but then we should not be able to conceive the elective will as free. Now this mutually opposed self-constraint and the inevitability of it makes us recognize the incomprehensible property of freedom.
The impulses of nature, then, contain hindrances to the fulfilment of duty in the mind of man, and resisting forces, some of them powerful; and he must judge himself able to combat these and to conquer them by means of reason, not in the future, but in the present, simultaneously with the thought; he must judge that he can do what the law unconditionally commands that be ought.
Now the power and resolved purpose to resist a strong but unjust opponent is called fortitude (fortitudo), and when concerned with the opponent of the moral character within us, it is virtue (virtus, fortitudo moralis). Accordingly, general deontology, in that part which brings not external, but internal, freedom under laws is the doctrine of virtue.
Jurisprudence had to do only with the formal condition of external freedom (the condition of consistency with itself, if its maxim became a universal law), that is, with law. Ethics, on the contrary, supplies us with a matter (an object of the free elective will), an end of pure reason which is at the same time conceived as an objectively necessary end, i.e., as duty for all men. For, as the sensible inclinations mislead us to ends (which are the matter of the elective will) that may contradict duty, the legislating reason cannot otherwise guard against their influence than by an opposite moral end, which therefore must be given a priori independently on inclination.
An end is an object of the elective will (of a rational being) by the idea of which this will is determined to an action for the production of this object. Now I may be forced by others to actions which are directed to an end as means, but I cannot be forced to have an end; I can only make something an end to myself. If, however, I am also bound to make something which lies in the notions of practical reason an end to myself, and therefore besides the formal determining principle of the elective will (as contained in law) to have also a material principle, an end which can be opposed to the end derived from sensible impulses; then this gives the notion of an end which is in itself a duty. The doctrine of this cannot belong to jurisprudence, but to ethics, since this alone includes in its conception self-constraint according to moral laws.
For this reason, ethics may also be defined as the system of the ends of the pure practical reason. The two parts of moral philosophy are distinguished as treating respectively of ends and of duties of constraint. That ethics contains duties to the observance of which one cannot be (physically) forced by others, is merely the consequence of this, that it is a doctrine of ends, since to be forced to have ends or to set them before one's self is a contradiction.
Now that ethics is a doctrine of virtue (doctrina officiorum virtutis) follows from the definition of virtue given above compared with the obligation, the peculiarity of which has just been shown. There is in fact no other determination of the elective will, except that to an end, which in the very notion of it implies that I cannot even physically be forced to it by the elective will of others. Another may indeed force me to do something which is not my end (but only means to the end of another), but he cannot force me to make it my own end, and yet I can have no end except of my own making. The latter supposition would be a contradiction- an act of freedom which yet at the same time would not be free. But there is no contradiction in setting before one's self an end which is also a duty: for in this case I constrain myself, and this is quite consistent with freedom.[*] But how is such an end possible? That is now the question. For the possibility of the notion of the thing (viz., that it is not self-contradictory) is not enough to prove the possibility of the thing itself (the objective reality of the notion).
[*]The less a man can be physically forced, and the more he can be morally forced (by the mere idea of duty), so much the freer he is. The man, for example, who is of sufficiently firm resolution and strong mind not to give up an enjoyment which he has resolved on, however much loss is shown as resulting therefrom, and who yet desists from his purpose unhesitatingly, though very reluctantly, when he finds that it would cause him to neglect an official duty or a sick father; this man proves his freedom in the highest degree by this very thing, that he cannot resist the voice of duty.