The rule of the judgement according to laws of pure practical reason is this: ask yourself whether, if the action you propose were to take place by a law of the system of nature of which you were yourself a part, you could regard it as possible by your own will. Everyone does, in fact, decide by this rule whether actions are morally good or evil. Thus, people say: "If everyone permitted himself to deceive, when he thought it to his advantage; or thought himself justified in shortening his life as soon as he was thoroughly weary of it; or looked with perfect indifference on the necessity of others; and if you belonged to such an order of things, would you do so with the assent of your own will?" Now everyone knows well that if he secretly allows himself to deceive, it does not follow that everyone else does so; or if, unobserved, he is destitute of compassion, others would not necessarily be so to him; hence, this comparison of the maxim of his actions with a universal law of nature is not the determining principle of his will. Such a law is, nevertheless, a type of the estimation of the maxim on moral principles. If the maxim of the action is not such as to stand the test of the form of a universal law of nature, then it is morally impossible. This is the judgement even of common sense; for its ordinary judgements, even those of experience, are always based on the law of nature. It has it therefore always at hand, only that in cases where causality from freedom is to be criticised, it makes that law of nature only the type of a law of freedom, because, without something which it could use as an example in a case of experience, it could not give the law of a pure practical reason its proper use in practice. It is therefore allowable to use the system of the world of sense as the type of a supersensible system of things, provided I do not transfer to the latter the intuitions, and what depends on them, but merely apply to it the form of law in general (the notion of which occurs even in the commonest use of reason, but cannot be definitely known a priori for any other purpose than the pure practical use of reason); for laws, as such, are so far identical, no matter from what they derive their determining principles. {BOOK_1|CHAPTER_2 ^paragraph 50}