made of them before. Such a transition necessitates a comparison of
the old and the new usage, in order to distinguish well the new path
from the old one and, at the same time, to allow their connection to
be observed. Accordingly considerations of this kind, including
those which are once more directed to the concept of freedom in the
practical use of the pure reason, must not be regarded as an
interpolation serving only to fill up the gaps in the critical
system of speculative reason (for this is for its own purpose
complete), or like the props and buttresses which in a hastily
constructed building are often added afterwards; but as true members
which make the connexion of the system plain, and show us concepts,
here presented as real, which there could only be presented
problematically. This remark applies especially to the concept of
freedom, respecting which one cannot but observe with surprise that so
many boast of being able to understand it quite well and to explain
its possibility, while they regard it only psychologically, whereas if
they had studied it in a transcendental point of view, they must
have recognized that it is not only indispensable as a problematical
concept, in the complete use of speculative reason, but also quite
incomprehensible; and if they afterwards came to consider its
practical use, they must needs have come to the very mode of
determining the principles of this, to which they are now so loth to
assent. The concept of freedom is the stone of stumbling for all
empiricists, but at the same time the key to the loftiest practical
principles for critical moralists, who perceive by its means that they
must necessarily proceed by a rational method. For this reason I beg
the reader not to pass lightly over what is said of this concept at
the end of the Analytic.

I must leave it to those who are acquainted with works of this