the will does not give itself the law, but is given by a foreign
impulse by means of a particular natural constitution of the subject
adapted to receive it. An absolutely good will, then, the principle of
which must be a categorical imperative, will be indeterminate as
regards all objects and will contain merely the form of volition
generally, and that as autonomy, that is to say, the capability of the
maxims of every good will to make themselves a universal law, is
itself the only law which the will of every rational being imposes
on itself, without needing to assume any spring or interest as a
foundation.

How such a synthetical practical a priori proposition is possible,
and why it is necessary, is a problem whose solution does not lie
within the bounds of the metaphysic of morals; and we have not here
affirmed its truth, much less professed to have a proof of it in our
power. We simply showed by the development of the universally received
notion of morality that an autonomy of the will is inevitably
connected with it, or rather is its foundation. Whoever then holds
morality to be anything real, and not a chimerical idea without any
truth, must likewise admit the principle of it that is here
assigned. This section then, like the first, was merely analytical.
Now to prove that morality is no creation of the brain, which it
cannot be if the categorical imperative and with it the autonomy of
the will is true, and as an a priori principle absolutely necessary,
this supposes the possibility of a synthetic use of pure practical
reason, which however we cannot venture on without first giving a
critical examination of this faculty of reason. In the concluding
section we shall give the principal outlines of this critical
examination as far as is sufficient for our purpose.