a mode or
capacity of perceiving. To perceive colours implies a capacity for
seeing; to hear noises implies a capacity for hearing. And these
capacities may fairly be called forms of perception. As soon as this
is realized, the conclusion is inevitable that Kant was led to think
of space and time as the only forms of perception, because in this
connexion he was thinking of each as a form of phenomena, i. e. as
something in which all bodies or their states are, or, from the point
of view of our knowledge, as that in which sensuous material is to be
arranged; for there is nothing except space and time in which such
arrangement could plausibly be said to be carried out.

As has been pointed out, Kant's argument falls into two main parts,
one of which prepares the way for the other. The aim of the former is
to show _firstly_ that our apprehension of space is _a priori_, and
_secondly_ that it belongs to perception and not to conception. The
aim of the latter is to conclude from these characteristics of our
apprehension of space that space is a property not of things in
themselves but only of phenomena. These arguments may be considered in
turn.

The really valid argument adduced by Kant for the _a priori_ character
of our apprehension of space is based on the nature of geometrical
judgements. The universality of our judgements in geometry is not
based upon experience, i. e. upon the observation of individual things
in space. The necessity of geometrical relations is apprehended
directly in virtue of the mind's own apprehending nature.
Unfortunately in the present context Kant ignores this argument and
substitutes two others, both of which are invalid.

1. "Space is no empirical conception[14] which has been derived from
external[15] experiences. For in order that certain sensations may be
related to something external to me (that is, to something in a
different part of space from that in which I am), in like manner, in
order that I may represent them as external to and next to each other,
and consequently as not merely different but as in different places,
the representation of space must already exist as a foundation.
Consequently, the representation of space cannot be borrowed from the
relations of external phenomena through experience; but, on the
contrary, this external experience is itself first possible only
through the said representation."[16] Here Kant is thinking that in
order to apprehend, for example, that A