But, that pure
reason without the admixture of any empirical principle is practical
of itself, this could only be shown from the commonest practical use
of reason, by verifying the fact, that every man's natural reason
acknowledges the supreme practical principle as the supreme law of his
will- a law completely a priori and not depending on any sensible
data. It was necessary first to establish and verify the purity of its
origin, even in the judgement of this common reason, before science
could take it in hand to make use of it, as a fact, that is, prior
to all disputation about its possibility, and all the consequences
that may be drawn from it. But this circumstance may be readily
explained from what has just been said; because practical pure
reason must necessarily begin with principles, which therefore must be
the first data, the foundation of all science, and cannot be derived
from it. It was possible to effect this verification of moral
principles as principles of a pure reason quite well, and with
sufficient certainty, by a single appeal to the judgement of common
sense, for this reason, that anything empirical which might slip
into our maxims as a determining principle of the will can be detected
at once by the feeling of pleasure or pain which necessarily
attaches to it as exciting desire; whereas pure practical reason
positively refuses to admit this feeling into its principle as a
condition.
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