imperative of the prohibition is categorical; then we cannot show with
certainty in any example that the will was determined merely by the
law, without any other spring of action, although it may appear to
be so. For it is always possible that fear of disgrace, perhaps also
obscure dread of other dangers, may have a secret influence on the
will. Who can prove by experience the non-existence of a cause when
all that experience tells us is that we do not perceive it? But in
such a case the so-called moral imperative, which as such appears to
be categorical and unconditional, would in reality be only a pragmatic
precept, drawing our attention to our own interests and merely
teaching us to take these into consideration.

We shall therefore have to investigate a priori the possibility of a
categorical imperative, as we have not in this case the advantage of
its reality being given in experience, so that [the elucidation of]
its possibility should be requisite only for its explanation, not
for its establishment. In the meantime it may be discerned
beforehand that the categorical imperative alone has the purport of
a practical law; all the rest may indeed be called principles of the
will but not laws, since whatever is only necessary for the attainment
of some arbitrary purpose may be considered as in itself contingent,
and we can at any time be free from the precept if we give up the
purpose; on the contrary, the unconditional command leaves the will no
liberty to choose the opposite; consequently it alone carries with
it that necessity which we require in a law.

Secondly, in the case of this categorical imperative or law of
morality, the difficulty (of discerning its possibility) is a very
profound one. It is an a priori synthetical practical proposition; *
and as there is so much difficulty in discerning the possibility of