admit that if a law is to have moral force, i.e., to be the basis of
an obligation, it must carry with it absolute necessity; that, for
example, the precept, "Thou shalt not lie," is not valid for men
alone, as if other rational beings had no need to observe it; and so
with all the other moral laws properly so called; that, therefore, the
basis of obligation must not be sought in the nature of man, or in the
circumstances in the world in which he is placed, but a priori
simply in the conception of pure reason; and although any other
precept which is founded on principles of mere experience may be in
certain respects universal, yet in as far as it rests even in the
least degree on an empirical basis, perhaps only as to a motive,
such a precept, while it may be a practical rule, can never be
called a moral law.

Thus not only are moral laws with their principles essentially
distinguished from every other kind of practical knowledge in which
there is anything empirical, but all moral philosophy rests wholly
on its pure part. When applied to man, it does not borrow the least
thing from the knowledge of man himself (anthropology), but gives laws
a priori to him as a rational being. No doubt these laws require a
judgement sharpened by experience, in order on the one hand to
distinguish in what cases they are applicable, and on the other to
procure for them access to the will of the man and effectual influence
on conduct; since man is acted on by so many inclinations that, though
capable of the idea of a practical pure reason, he is not so easily
able to make it effective in concreto in his life.

A metaphysic of morals is therefore indispensably necessary, not
merely for speculative reasons, in order to investigate the sources of
the practical principles which are to be found a priori in our reason,