priori, how a mere thought, which itself contains nothing sensible,
can itself produce a sensation of pleasure or pain; for this is a
particular kind of causality of which as of every other causality we
can determine nothing whatever a priori; we must only consult
experience about it. But as this cannot supply us with any relation of
cause and effect except between two objects of experience, whereas
in this case, although indeed the effect produced lies within
experience, yet the cause is supposed to be pure reason acting through
mere ideas which offer no object to experience, it follows that for us
men it is quite impossible to explain how and why the universality
of the maxim as a law, that is, morality, interests. This only is
certain, that it is not because it interests us that it has validity
for us (for that would be heteronomy and dependence of practical
reason on sensibility, namely, on a feeling as its principle, in which
case it could never give moral laws), but that it interests us because
it is valid for us as men, inasmuch as it had its source in our will
as intelligences, in other words, in our proper self, and what belongs
to mere appearance is necessarily subordinated by reason to the nature
of the thing in itself.

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.