ies that space is an illusion,
inasmuch as it is not a real property of things at all. This
implication, however, is precisely the conclusion which Kant wishes to
avoid. He takes infinite trouble to explain that he does not hold
space and time to be illusions.[3] Though _transcendentally ideal_
(i. e. though they do not belong to things in themselves), they are
_empirically real_. In other words, space and time are real relations
of _something_, though not of things in themselves.

[3] B. 44, 52, 53-4, 62-3, 69-70; M. 27, 31-2, 37-8, 41-2;
_Prol._, Sec. 13, Remark iii.

How, then, does Kant obtain something of which space and time can be
regarded as really relations? He reaches it by a transition which at
first sight seems harmless. In stating the fact of perception he
substitutes for the assertion that things appear so and so to us the
assertion that things produce appearances in us. In this way, instead
of an assertion which relates to the thing and states what it is not
but only appears, he obtains an assertion which introduces a second
reality distinct from the thing, viz. an appearance or phenomenon, and
thereby he gains something other than the thing to which space can be
attached as a real predicate. He thus gains something in respect of
which, with regard to spatial relations we can be said to have
_knowledge_ and not illusion. For the position now is that space,
though not a property of things in themselves, _is_ a property of
phenomena or appearances; in other words, that while things in
themselves are not spatial, phenomena and appearances _are_ spatial.
As evidence of this transition, it is enough to point out that, while
he states the _problem_ in the form 'Are things in themselves spatial
or are they only spatial as appearing to us?'[4] he usually states the
_conclusion_ in the form 'Space is the form of phenomena', i. e.
phenomena are spatial. A transition is thereby implied from 'things as
appearing' to 'appearances'. At the same time, it is clear that Kant
is not aware of the transition, but considers the expressions
equivalent, or, in other words, fails to distinguish them. For both
modes of stating the conclusion are to be found even in the same
sentence. "This predicate [space] is applied to things only in so far
as they appear to us, i. e. are objects of sensibility [i. e.
phenomena]."[5] Again, the common phrase 'things as phenomena' implies
the same confusion. Moreover, if Kant had reali