that the categorical imperative alone has the purport of a practical law; all the rest may indeed be called principles of the will but not laws, since whatever is only necessary for the attainment of some arbitrary purpose may be considered as in itself contingent, and we can at any time be free from the precept if we give up the purpose; on the contrary, the unconditional command leaves the will no liberty to choose the opposite; consequently it alone carries with it that necessity which we require in a law.

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,