209-10, M. 127.

In other words, 'We can lay down _a priori_ that all sensations have a
certain degree of intensity, and that between a sensation of a given
intensity and the total absence of sensation there is possible an
infinite number of sensations varying in intensity from nothing to
that degree of intensity. Therefore the real, which corresponds to
sensation, can also be said _a priori_ to admit of an infinite variety
of degree.'

Though the principle established is of little intrinsic importance,
the account of it is noticeable for two reasons. In the first place,
although Kant clearly means by the 'real corresponding to sensation' a
body in space, and regards it as a phenomenon, it is impossible to see
how he can avoid the charge that he in fact treats it as a thing in
itself.[8] For the correspondence must consist in the fact that the
real causes or excites sensation in us, and therefore the real, i. e.
a body in space, is implied to be a thing in itself. In fact, Kant
himself speaks of considering the real in the phenomenon as the cause
of sensation,[9] and, in a passage added in the second edition, after
proving that sensation must have an intensive quantity, he says that,
corresponding to the intensive quantity of sensation, an intensive
quantity, i. e. _a degree of influence on sense_, must be attributed
to all objects of sense-perception.[10] The difficulty of consistently
maintaining that the real, which corresponds to sensation, is a
phenomenon is, of course, due to the impossibility of distinguishing
between reality and appearance within phenomena.[11]

[8] Cf. p. 257 note.

[9] B. 210, M. 128.

[10] B. 208, M. 126. The italics are mine. Cf. from the same
passage, "Phenomena contain, over and above perception, the
materials for some object (through which is represented
something existing in space and time), i. e. they contain the
real of sensation as a merely subjective representation of
which we can only become conscious that _the subject is
affected_, and which we relate _to an object in general_."
(The italics are mine.)

[11] Cf. pp. 94-100.

In the second place, Kant expressly allows that in this anticipation
we succeed in discovering _a priori_ a characteristic of sensation,
although sensation constitutes that empirical element in phenomena,
which on Kant's general view cannot be apprehended _a priori_.

"Nevertheless, this anticipation of se