Fourthly, as regards meritorious duties towards others:  the natural end which all men have in their own happiness.  Now humanity might indeed subsist, although no one should contribute anything to the happiness of others, provided he did not intentionally withdraw anything from it; but after all, this would only harmonise negatively not positively with humanity as an end in itself, if everyone does not also endeavor, as far as in him lies, to forward the ends of others.  For the ends of any subject which is an end in himself, ought as far as possible to be my ends also, if that conception is to have its full effect with me.

This principle, that humanity and generally every rational nature is an end in itself (which is the supreme limiting condition of every man’s freedom of action), is not borrowed from experience, firstly, because it is universal, applying as it does to all rational beings whatever, and experience is not capable of determining anything about them; secondly, because it does not present humanity as an end to men (subjectively), that is as an object which men do of themselves actually adopt as an end; but as an objective end, which must as a law constitute the supreme limiting condition of all our subjective ends, let them be what we will; it must therefore spring from pure, reason.  In fact the objective principle of all practical legislation lies (according to the first principle) in the rule and its form of universality which makes it capable of being a law (say, e. g., a law of nature); but the subjective principle is in the end; now by the second principle the subject of all ends is each rational being, inasmuch as it is an end in itself.  Hence follows the third practical principle of the will, which is the ultimate condition of its harmony with the universal practical reason, viz.:  the idea of the will of every rational being as a universally legislative will.

On this principle all maxims are rejected which are inconsistent with the will being itself universal legislator.  Thus the will is not subject simply to the law, but so subject that it must be regarded as itself giving the law, and on this ground only, subject to the law (of which it can regard itself as the author).

In the previous imperatives, namely, that based on the conception of the conformity of actions to general laws, as in a physical system of nature, and that based on the universal prerogative of rational beings as ends in themselves—­these imperatives just because they were conceived as categorical, excluded from any share in their authority all admixture of any interest as a spring of action; they were however only assumed to be categorical, because such an assumption was necessary to explain the conception of duty.  But we could not prove independently that there are practical propositions which command categorically, nor can it be proved in this section; one thing however could be done, namely, to indicate in the imperative itself by some determinate expression, that in the case of volition from duty all interest is renounced, which is the specific criterion of categorical as distinguished from hypothetical imperatives.  This is done in the present (third) formula of the principle, namely, in the idea of the will of every rational being as a universally legislating will.