ean that when sensation
arises, the understanding judges that there is something causing it;
and this assertion must really be _a priori_, because not dependent
upon experience. Unfortunately the two statements so interpreted are
wholly inconsistent with the account of the functions of the
sensibility and the understanding which has just been quoted.

[8] See p. 29, note 1.

[9] Cf. B. 1, M. 1.

Further, this theory of perception has two forms. In its first form
the theory is physical rather than metaphysical, and is based upon
our possession of physical organs. It assumes that the reality to be
apprehended is the world of space and time, and it asserts that by the
action of bodies upon our physical organs our sensibility is affected,
and that thereby sensations are originated in us. Thereupon a problem
arises. For if the contribution of the sensibility to our knowledge
of the physical world is limited to a succession of sensations,
explanation must be given of the fact that we have succeeded with
an experience confined to these sensations in acquiring knowledge
of a world which does not consist of sensations.[10] Kant, in fact,
in the _Aesthetic_ has this problem continually before him, and
tries to solve it. He holds that the mind, by means of its forms of
perception and its conceptions of the understanding, superinduces upon
sensations, as data, spatial and other relations, in such a way that
it acquires knowledge of the spatial world.

[10] Cf. B. 1 init., M. 1 init.; B. 34, M. 21 sub fin.

An inherent difficulty, however, of this 'physical' theory of
perception leads to a transformation of it. If, as the theory
supposes, the cause of sensation is outside or beyond the mind, it
cannot be known. Hence the initial assumption that this cause is the
physical world has to be withdrawn, and the cause of sensation comes
to be thought of as the thing in itself of which we can know nothing.
This is undoubtedly the normal form of the theory in Kant's mind.

It may be objected that to attribute to Kant at any time the physical
form of the theory is to accuse him of an impossibly crude confusion
between things in themselves and the spatial world, and that he can
never have thought that the cause of sensation, being as it is outside
the mind, is spatial. But the answer is to be found in the fact that
the problem just referred to as occupying Kant's attention in the
_Aesthetic_ is only a problem at all so long