noumenon

an sich, noumenon puhekieltä In the philosophy of w:Immanuel_Kant Immanuel Kant (1724-1804) and those whom he influenced, a thing as it is independent of any conceptualization or perception by the human mind; a thing-in-itself, postulated by practical reason but existing in a condition which is in principle unknowable and unexperienceable.
1871, David Asher, "Schopenhauer and Darwinism," Journal of Anthropology, vol. 1, no. 3 (Jan), page 317:

The final result of Kant's philosophy, expressed in the concisest terms, was the proposition, so humiliating to human cognition, but, at the same time, so fertile in consequences, that we can know only phenomena, or the outward appearances of things, but not the noumenon, or the thing in itself.

We have no specific concept of the noumenon, but think of it merely as whatever the object may be apart from the manner in which our knowledge exhibits it.

That, we have seen, is what prevents the two truths from collapsing into an appearance/reality or phenomenon/noumenon distinction.