do except in one passage,[1] where the
argument fails. The first two points established, Kant is able to
develop his main thesis, viz. that it is a condition of the validity
of the _a priori_ judgements which relate to space and time that these
are characteristics of phenomena, and not of things in themselves.

[1] B. 58, M. 35.

It will be convenient to consider his treatment of space and time
separately, and to begin with his treatment of space. It is necessary,
however, first of all to refer to the term 'form of perception'. As
Kant conceives a form of perception, it involves three antitheses.

(1) As a _form_ of perception it is opposed, as a way or mode of
perceiving, to particular perceptions.

(2) As a form or mode of _perception_ it is opposed to a form or mode
of _conception_.

(3) As a form of _perception_ it is also opposed, as a way in which we
apprehend things, to a way in which things are.

While we may defer consideration of the second and third antitheses,
we should at once give attention to the nature of the first, because
Kant confuses it with two other antitheses. There is no doubt that in
general a _form_ of perception means for Kant a general capacity of
perceiving which, as such, is opposed to the actual perceptions in
which it is manifested. For according to him our spatial perceptions
are not foreign to us, but manifestations of our general perceiving
nature; and this view finds expression in the assertion that space is
a form of perception or of sensibility.[2]

[2] Cf. B. 43 init., M. 26 med.

Unfortunately, however, Kant frequently speaks of this form of
perception as if it were the same thing as the actual perception of
empty space.[3] In other words, he implies that such a perception is
possible, and confuses it with a potentiality, i. e. the power of
perceiving that which is spatial. The confusion is possible because it
can be said with some plausibility that a perception of empty
space--if its possibility be allowed--does not inform us about actual
things, but only informs us what must be true of things, if there
prove to be any; such a perception, therefore, can be thought of as a
possibility of knowledge rather than as actual knowledge.

[3] e. g. B. 34, 35, M. 22; B. 41, M. 25; _Prol._ Secs. 9-11.
The commonest expression of the confusion is to be found in
the repeated assertion that space is a pure perception.

The second confusion is closely related to