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 causality 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 an 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