So far, we have shown that duties must be based on a categorical rather than a hypothetical imperative, and we have established the content of the one and only categorical imperative. We have yet to establish conclusively that the categorical imperative is a binding law for any rational being possessing a free will.
If there is some necessary law that compels rational beings to follow the categorical imperative, that law must be based on the concept of the "will" of a rational being. The "will" is the faculty that enables rational beings to choose what course of action to follow. Rational beings may pursue certain "ends" using appropriate "means." Ends that are based on physical needs or wants will always provide merely hypothetical imperatives. The categorical imperative, however, may be based only on something that is an "end in itself"-- that is, an end that is a means only to itself and not to some other need, desire, or purpose.
Rational beings are ends in themselves. In pursuing their objectives, rational beings must always view themselves not only as means to some purpose, but also as ends in themselves. They must also recognize that other rational beings are ends in themselves as well. Thus if we formulate the categorical imperative in terms of the will of a rational being, it would run as follows: act in such a way that you always treat other people not merely as means to some end, but also as ends in themselves.
The four examples of duty that were discussed earlier are consistent with this formulation of the law. When people commit suicide, they treat their own life as a mere means for escaping an upsetting situation. When people make false promises to repay debts, they treat the people they have borrowed from as mere means to their own financial gain. A view of humanity as an end in itself requires us to pursue the maximum fulfillment of humanity's potential, which means that we must cultivate our talents. Similarly, a view of humanity as an end in itself requires us to work towards maximum happiness for humanity, which means that we must take care for the welfare of others.
The principle that every rational being is an end in itself is universal and applies to all rational beings. It comes from reason, not from experience. Now, if rational beings are ends in themselves, and not means to some other end, then the will of a rational being must be thought of as the maker of universal law. Otherwise their actions would be governed by some interest and they would function as mere means to some purpose. When rational beings will something for the sake of duty alone, they must renounce all interests and motivations other than duty. Thus their obedience to the law cannot be based on any specific interest. Rather, they must understand themselves to be subjects as well as authors of the law, and they must recognize that the law requires unconditional obedience.
This notion of rational beings as simultaneous authors and subjects of universal law leads us to the idea of a perfect community in which all people follow the objective laws of reason and treat their fellows not merely as means to ends but also always as ends in themselves. This perfect community may be called the "kingdom of ends," meaning a legal community (kingdom) composed of ends in themselves which respects all its members as ends in themselves. Morality consists in adopting only those maxims and motives that are consistent with the establishment of a kingdom of ends.