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

{Introduction ^paragraph 80}

(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.