Secondly, in the case of this categorical imperative or law of morality, the difficulty (of discerning its possibility) is a very profound one. It is an a priori synthetical practical proposition;*and as there is so much difficulty in discerning the possibility of speculative propositions of this kind, it may readily be supposed that the difficulty will be no less with the practical.
*I connect the act with the will without presupposing any condition resulting from any inclination, but a priori, and therefore necessarily (though only objectively, i.e., assuming the idea of a reason possessing full power over all subjective motives).
This is accordingly a practical proposition which does not deduce the willing of an action by mere analysis from another already presupposed (for we have not such a perfect will), but connects it immediately with the conception of the will of a rational being, as something not contained in it.
In this problem we will first inquire whether the mere conception of a categorical imperative may not perhaps supply us also with the formula of it, containing the proposition which alone can be a categorical imperative; for even if we know the tenor of such an absolute command, yet how it is possible will require further special and laborious study, which we postpone to the last section.
When I conceive a hypothetical imperative, in general I do not know beforehand what it will contain until I am given the condition.
But when I conceive a categorical imperative, I know at once what it contains. For as the imperative contains besides the law only the necessity that the maxims* shall conform to this law, while the law contains no conditions restricting it, there remains nothing but the general statement that the maxim of the action should conform to a universal law, and it is this conformity alone that the imperative properly represents as necessary.
*A maxim is a subjective principle of action, and must be distinguished from the objective principle, namely, practical law. The former contains the practical rule set by reason according to the conditions of the subject (often its ignorance or its inclinations), so that it is the principle on which the subject acts; but the law is the objective principle valid for every rational being, and is the principle on which it ought to act that is an imperative.
There is therefore but one categorical imperative, namely, this: Act only on that maxim whereby thou canst at the same time will that it should become a universal law.
Now if all imperatives of duty can be deduced from this one imperative as from their principle, then, although it should remain undecided what is called duty is not merely a vain notion, yet at least we shall be able to show what we understand by it and what this notion means.
Since the universality of the law according to which effects are produced constitutes what is properly called nature in the most general sense (as to form), that is the existence of things so far as it is determined by general laws, the imperative of duty may be expressed thus: Act as if the maxim of thy action were to become by thy will a universal law of nature.
We will now enumerate a few duties, adopting the usual division of them into duties to ourselves and ourselves and to others, and into perfect and imperfect duties.**It must be noted here that I reserve the division of duties for a future metaphysic of morals; so that I give it here only as an arbitrary one (in order to arrange my examples). For the rest, Iunderstand by a perfect duty one that admits no exception in favour of inclination and then I have not merely external but also internal perfect duties. This is contrary to the use of the word adopted in the schools; but I do not intend to justify there, as it is all one for my purpose whether it is admitted or not.
1. A man reduced to despair by a series of misfortunes feels wearied of life, but is still so far in possession of his reason that he can ask himself whether it would not be contrary to his duty to himself to take his own life. Now he inquires whether the maxim of his action could become a universal law of nature. His maxim is: "From self-love I adopt it as a principle to shorten my life when its longer duration is likely to bring more evil than satisfaction." It is asked then simply whether this principle founded on self-love can become a universal law of nature. Now we see at once that a system of nature of which it should be a law to destroy life by means of the very feeling whose special nature it is to impel to the improvement of life would contradict itself and, therefore, could not exist as a system of nature; hence that maxim cannot possibly exist as a universal law of nature and, consequently, would be wholly inconsistent with the supreme principle of all duty.
