This separation on which the subdivision of moral philosophy in
general rests, is founded on this: that the notion of freedom, which
is common to both, makes it necessary to divide duties into those of
external and those of internal freedom; the latter of which alone
are ethical. Hence this internal freedom which is the condition of all
ethical duty must be discussed as a preliminary (discursus
praeliminaris), just as above the doctrine of conscience was discussed
as the condition of all duty.
{INTRODUCTION ^paragraph 185}