The speculative employment of reason with respect to nature leads to
the absolute necessity of some supreme cause of the world: the
practical employment of reason with a view to freedom leads also to
absolute necessity, but only of the laws of the actions of a
rational being as such. Now it is an essential principle of reason,
however employed, to push its knowledge to a consciousness of its
necessity (without which it would not be rational knowledge). It is
however an equally essential restriction of the same reason that it
can neither discern the necessity of what is or what happens, nor of
what ought to happen, unless a condition is supposed on which it is
or happens or ought to happen. In this way, however, by the constant
inquiry for the condition, the satisfaction of reason is only
further and further postponed. Hence it unceasingly seeks the
unconditionally necessary, and finds itself forced to assume it,
although without any means of making it comprehensible to itself,
happy enough if only it can discover a conception which agrees with
this assumption. It is therefore no fault in our deduction of the
supreme principle of morality, but an objection that should be made
to human reason in general, that it cannot enable us to conceive the
absolute necessity of an unconditional practical law (such as the
categorical imperative must be). It cannot be blamed for refusing to
explain this necessity by a condition, that is to say, by means of
some interest assumed as a basis, since the law would then cease to
be a moral law, i. e. a supreme law of freedom. And thus while we do
not comprehend the practical unconditional necessity of the moral
imperative, we yet comprehend its incomprehensibility, and this is
all that can be fairly demanded of a philosophy which strives to
carry its principles up to the very limit of human reason.