The question then:  How a categorical imperative is possible can be answered to this extent that we can assign the only hypothesis on which it is possible, namely, the idea of freedom; and we can also discern the necessity of this hypothesis, and this is sufficient for the practical exercise of reason, that is, for the conviction of the validity of this imperative, and hence of the moral law; but how this hypothesis itself is possible can never be discerned by any human reason.  On the hypothesis, however, that the will of an intelligence is free, its autonomy, as the essential formal condition of its determination, is a necessary consequence.  Moreover, this freedom of will is not merely quite possible as a hypothesis (not involving any contradiction to the principle of physical necessity in the connexion of the phenomena of the sensible world) as speculative philosophy can show:  but further, a rational being who is conscious of a causality [Footnote:  Reading “einer” for “seiner.”] through reason, that is to say, of a will (distinct from desires), must of necessity make it practically, that is, in idea, the condition of all his voluntary actions.  But to explain how pure reason can be of itself practical without the aid of any spring of action that could be derived from any other source, i. e. how the mere principle of the universal validity of all its maxims as laws (which would certainly be the form of a pure practical reason) can of itself supply a spring, without any matter (object) of the will in which one could antecedently take any interest; and how it can produce an interest which would be called purely moral; or in other words, how pure reason can be practical—­to explain this is beyond the power of human reason, and all the labour and pains of seeking an explanation of it are lost.

It is just the same as if I sought to find out how freedom itself is possible as the causality of a will.  For then I quit the ground of philosophical explanation, and I have no other to go upon.  I might indeed revel in the world of intelligences which still remains to me, but although I have an idea of it which is well founded, yet I have not the least knowledge of it, nor can I ever attain to such knowledge with all the efforts of my natural faculty of reason.  It signifies only a something that remains over when I have eliminated everything belonging to the world of sense from the actuating principles of my will, serving merely to keep in bounds the principle of motives taken from the field of sensibility; fixing its limits and showing that it does not contain all in all within itself, but that there is more beyond it; but this something more I know no further.  Of pure reason which frames this ideal, there remains after the abstraction of all matter, i. e., knowledge of objects, nothing but the form, namely, the practical law of the universality of the maxims, and in conformity with this the conception of reason in reference to a pure world of understanding as a possible efficient cause, that is a cause determining the will.  There must here be a total absence of springs; unless this idea of an intelligible world is itself the spring, or that in which reason primarily takes an interest; but to make this intelligible is precisely the problem that we cannot solve.