Geometry is a science which determines the properties of space
synthetically, and yet a priori. What, then, must be our
representation of space, in order that such a cognition of it may be
possible? It must be originally intuition, for from a mere conception,
no propositions can be deduced which go out beyond the conception,
and yet this happens in geometry. (Introd. V.) But this intuition must
be found in the mind a priori, that is, before any perception of
objects, consequently must be pure, not empirical, intuition. For
geometrical principles are always apodeictic, that is, united with
the consciousness of their necessity, as: "Space has only three
dimensions." But propositions of this kind cannot be empirical
judgements, nor conclusions from them. (Introd. II.) Now, how can an
external intuition anterior to objects themselves, and in which our
conception of objects can be determined a priori, exist in the human
mind? Obviously not otherwise than in so far as it has its seat in
the subject only, as the formal capacity of the subject's being affected
by objects, and thereby of obtaining immediate representation, that
is, intuition; consequently, only as the form of the external sense
in general.

Thus it is only by means of our explanation that the possibility
of geometry, as a synthetical science a priori, becomes
comprehensible. Every mode of explanation which does not show us
this possibility, although in appearance it may be similar to ours,
can with the utmost certainty be distinguished from it by these marks.



SS 4. Conclusions from the foregoing Conceptions.