ception, as such, involves the actual presence
of an object; yet the pure perception of space involved by
geometry--which, as pure, is the perception of empty space, and which,
as the perception of empty space, is _a priori_ in the sense of
temporally prior to the perception of actual objects--presupposes that
an object is not actually present.

[39] _Prol._ Sec. 8.

The solution is given in the next section. "Were our perception
necessarily of such a kind as to represent things _as they are in
themselves_, no perception would take place _a priori_, but would
always be empirical. For I can only know what is contained in the
object in itself, if it is present and given to me. No doubt it is
even then unintelligible how the perception of a present thing should
make me know it as it is in itself, since its qualities cannot migrate
over into my faculty of representation; but, even granting this
possibility, such a perception would not occur _a priori_, i. e.
before the object was presented to me; for without this presentation,
no basis of the relation between my representation and the object can
be imagined; the relation would then have to rest upon inspiration. It
is therefore possible only in one way for my perception to precede the
actuality of the object and to take place as _a priori_ knowledge,
viz. _if it contains nothing but the form of the sensibility, which
precedes in me, the subject, all actual impressions through which I am
affected by objects_. For I can know _a priori_ that objects of the
senses can only be perceived in accordance with this form of the
sensibility. Hence it follows that propositions which concern merely
this form of sensuous perception will be possible and valid for
objects of the senses, and in the same way, conversely, that
perceptions which are possible _a priori_ can never concern any things
other than objects of our senses."

This section clearly constitutes the turning-point in Kant's argument,
and primarily expresses, in an expanded form, the central doctrine of
Sec. 3 of the _Aesthetic_, that an external perception anterior to
objects themselves, and in which our conceptions of objects can be
determined _a priori_, is possible, if, and only if, it has its seat
in the subject as its formal nature of being affected by objects, and
consequently as the form of the external sense in general. It argues
that, since this is true, and since geometrical judgements involve
such a percept