“The concept of a noumenon is a merely limiting concept, the function of which is to curb the pretensions of sensibility; and it is therefore only of negative employment. At the same time it is no arbitrary invention, and it is bound up with the limitation of sensibility, though it cannot affirm anything positive beyond the field of sensibility.”[1299]
This confusion, between the concept of a noumenon and the less definite concept of object in general, which is probably due to the combining of manuscripts of different dates, is corrected in the second edition by means of a new distinction which Kant introduces, evidently for this very purpose. The term noumenon may, he there says,[1300] be used either positively or negatively. Taken positively, it signifies “an object of a non-sensuous intuition”; regarded negatively, it means only “a thing so far as it is not an object of our sensuous intuition.” Only in its negative employment, he states, is it required as a limiting concept; and it is then, as he recognises, indistinguishable from the notion of the unknown thing in itself.
But despite this variation in mode of expression, in the main Kant holds consistently to his fundamental teaching.
“...understanding is not limited through sensibility; on the contrary, it itself limits sensibility by applying the term noumena to things in themselves (things not regarded as appearances). But in so doing it at the same time sets limits to itself, recognising that it cannot know these noumena through any of the categories, and that it must therefore think them only under the title of an unknown something.”[1301]
Or as Kant adds in the concluding sentence of this chapter:
“...the problematic thought which leaves open a place for [intelligible objects], serves only, like an empty space, for the limitation of empirical principles, without itself containing or revealing any other object of knowledge beyond their sphere.”
A sentence in A 258 = B 314 deserves special notice.
“...we can never know whether such a transcendental or exceptional knowledge is possible under any conditions, least of all if it is to be regarded as of the sort that stands under our ordinary categories.”
This sentence clearly shows that Kant was willing to recognise that the categories may be inapplicable, not merely owing to lack of data for their specification, but because of their inherent character. They may be intrinsically inapplicable, expressing only the modi of our self-consciousness. They may be merely the instruments of our human thinking, not forms necessary to knowledge as such.
RELEVANT PASSAGES IN THE SECTION ON AMPHIBOLY