(1) OUR OWN PERFECTION as an end which is also a duty
(a) Physical perfection; that is, cultivation of all our faculties generally for the promotion of the ends set before us by reason. That this is a duty, and therefore an end in itself, and that the effort to effect this even without regard to the advantage that it secures us, is based, not on a conditional (pragmatic), but an unconditional (moral) imperative, may be seen from the following consideration. The power of proposing to ourselves an end is the characteristic of humanity (as distinguished from the brutes). With the end of humanity in our own person is therefore combined the rational will, and consequently the duty of deserving well of humanity by culture generally, by acquiring or advancing the power to carry out all sorts of possible ends, so far as this power is to be found in man; that is, it is a duty to cultivate the crude capacities of our nature, since it is by that cultivation that the animal is raised to man, therefore it is a duty in itself.
This duty, however, is merely ethical, that is, of indeterminate obligation. No principle of reason prescribes how far one must go in this effort (in enlarging or correcting his faculty of understanding, that is, in acquisition of knowledge or technical capacity); and besides the difference in the circumstances into which men may come makes the choice of the kind of employment for which he should cultivate his talent very arbitrary. Here, therefore, there is no law of reason for actions, but only for the maxim of actions, viz.: "Cultivate thy faculties of mind and body so as to be effective for all ends that may come in thy way, uncertain which of them may become thy own."
(b) Cultivation of Morality in ourselves. The greatest moral perfection of man is to do his duty, and that from duty (that the law be not only the rule but also the spring of his actions). Now at first sight this seems to be a strict obligation, and as if the principle of duty commanded not merely the legality of every action, but also the morality, i.e., the mental disposition, with the exactness and strictness of a law; but in fact the law commands even here only the maxim of the action, namely, that we should seek the ground of obligation, not in the sensible impulses (advantage or disadvantage), but wholly in the law; so that the action itself is not commanded. For it is not possible to man to see so far into the depth of his own heart that he could ever be thoroughly certain of the purity of his moral purpose and the sincerity of his mind even in one single action, although he has no doubt about the legality of it. Nay, often the weakness which deters a man from the risk of a crime is regarded by him as virtue (which gives the notion of strength). And how many there are who may have led a long blameless life, who are only fortunate in having escaped so many temptations. How much of the element of pure morality in their mental disposition may have belonged to each deed remains hidden even from themselves.
Accordingly, this duty to estimate the worth of one's actions not merely by their legality, but also by their morality (mental disposition), is only of indeterminate obligation; the law does not command this internal action in the human mind itself, but only the maxim of the action, namely, that we should strive with all our power that for all dutiful actions the thought of duty should be of itself an adequate spring.
(2) HAPPINESS OF OTHERS as an end which is also a duty
(a) Physical Welfare. Benevolent wishes may be unlimited, for they do not imply doing anything. But the case is more difficult with benevolent action, especially when this is to be done, not from friendly inclination (love) to others, but from duty, at the expense of the sacrifice and mortification of many of our appetites. That this beneficence is a duty results from this: that since our self-love cannot be separated from the need to be loved by others (to obtain help from them in case of necessity), we therefore make ourselves an end for others; and this maxim can never be obligatory except by having the specific character of a universal law, and consequently by means of a will that we should also make others our ends. Hence the happiness of others is an end that is also a duty.
I am only bound then to sacrifice to others a part of my welfare without hope of recompense: because it is my duty, and it is impossible to assign definite limits how far that may go. Much depends on what would be the true want of each according to his own feelings, and it must be left to each to determine this for himself. For that one should sacrifice his own happiness, his true wants, in order to promote that of others, would be a self-contradictory maxim if made a universal law. This duty, therefore, is only indeterminate; it has a certain latitude within which one may do more or less without our being able to assign its limits definitely. The law holds only for the maxims, not for definite actions.
(b) Moral well-being of others (salus moral is) also belongs to the happiness of others, which it is our duty to promote, but only a negative duty. The pain that a man feels from remorse of conscience, although its origin is moral, is yet in its operation physical, like grief, fear, and every other diseased condition. To take care that he should not be deservedly smitten by this inward reproach is not indeed my duty but his business; nevertheless, it is my duty to do nothing which by the nature of man might seduce him to that for which his conscience may hereafter torment him, that is, it is my duty not to give him occasion of stumbling. But there are no definite limits within which this care for the moral satisfaction of others must be kept; therefore it involves only an indeterminate obligation.