To prove the possibility of the Imperative of morality is more difficult.  As categorical, it presupposes nothing else to rest its necessity upon; while by way of experience, it can never be made out to be more than a prudential precept—­i.e., a pragmatic or hypothetic principle.  Its possibility must therefore be established a priori.  But the difficulty will then appear no matter of wonder, when it is remembered (from the Critique of Pure Reason) how hard it is to establish synthetic propositions a priori.

The question of the possibility, however, meanwhile postponed, the mere conception of a categorical Imperative is found to yield the one formula that can express it, from its not being dependent, like a hypothetical Imperative, on any external condition.  Besides the Law (or objective principle of conduct), the only thing implied in the Imperative being the necessity laid upon the Maxim (or subjective principle) to conform to the law—­a law limited by no condition; there is nothing for the maxim to be conformed to but the universality of a law in general, and it is the conformity alone that properly constitutes the Imperative necessary.  The Imperative is thus single, and runs:  Act according to that maxim only which you can wish at the same time to become a universal law.  Or, since universality of law as determining effects is what we understand by nature:  Act as if the maxim of your action ought by your will to become the universal law of nature.

Taking cases of duties according to the common divisions of duties to ourselves and to others, perfect and imperfect, he proceeds to show that they may be all deduced from the single Imperative; the question of the reality of duty, which is the same as the establishment of the possibility of the Imperative as a synthetic practical proposition a priori, at present altogether apart.  Suppose a man tempted to commit suicide, with the view of bettering his evil condition; but it is contradictory that the very principle of self-conservation should lead to self-destruction, and such a maxim of conduct cannot therefore become a universal law of nature.  Next, the case of a man borrowing without meaning to repay, has only to be turned into a universal law, and the thing becomes impossible; nobody would lend.  Again, to neglect a talent that is generally useful for mere ease and self-gratification, can indeed be supposed a universal practice, but can never be wished to be.  Finally, to refuse help to others universally might not ruin the race, but can be wished by no one that knows how soon he must himself need assistance.  Now, the rule was, that a maxim of conduct should be wished to become the universal law.  In the last two cases, it cannot be wished; in the others, the maxim cannot even be conceived in universal form.  Thus, two grades of duty, one admitting of merit, the other so strict as to be irremissible, are established on the general principle.  The principle is moreover confirmed in the case of transgression of duty:  the transgressor by no means wishes to have his act turned into a general rule, but only seeks special and temporary exemption from a law allowed by himself to be universal.