The autonomy of the will is the sole principle of all moral laws and
of all duties which conform to them; on the other hand, heteronomy
of the elective will not only cannot be the basis of any obligation,
but is, on the contrary, opposed to the principle thereof and to the
morality of the will.

{BOOK_1|CHAPTER_1 ^paragraph 85}

In fact the sole principle of morality consists in the
independence on all matter of the law (namely, a desired object),
and in the determination of the elective will by the mere universal
legislative form of which its maxim must be capable. Now this
independence is freedom in the negative sense, and this
self-legislation of the pure, and therefore practical, reason is
freedom in the positive sense. Thus the moral law expresses nothing
else than the autonomy of the pure practical reason; that is, freedom;
and this is itself the formal condition of all maxims, and on this
condition only can they agree with the supreme practical law. If
therefore the matter of the volition, which can be nothing else than
the object of a desire that is connected with the law, enters into the
practical law, as the condition of its possibility, there results
heteronomy of the elective will, namely, dependence on the physical
law that we should follow some impulse or inclination. In that case
the will does not give itself the law, but only the precept how
rationally to follow pathological law; and the maxim which, in such
a case, never contains the universally legislative form, not only
produces no obligation, but is itself opposed to the principle of a