 it also follows naturally from the concept of an appearance in general that something must correspond to it which is not in itself appearance, for appearance can be nothing for itself and outside of our kind of representation; thus, if there is not to be a constant circle, the word "appearance" must already indicate a relation to something the immediate representation of which is, to be sure, sensible, but which in itself, without this constitution of our sensibility (on which the form of our intuition is grounded), must be something, i.e., an object independent of sensibility. Now from this arises the concept of a noumenon, which, however, is not at all positive and does not signify a determinate cognition of something in general, in which I abstract from all form of sensible intuition. (A251–2)