Noumenon — In Immanuel Kant’s philosophy, the noumenon is the object as it would be in itself, independently of how it appears to human sensibility. The word comes from the Greek nooúmenon — “that which is thought” —, the participle of the verb noein (to think, to apprehend by reason), and it stands in direct opposition to the phenomenon, which designates the thing as it appears in experience, shaped by the forms of sensibility (space and time) and by the categories of the understanding. This distinction is central to the Critique of Pure Reason (1781): according to Kant, we know only phenomena, that is, objects as they are given to us in sensible intuition, never things as they are in themselves, which he calls the thing in itself (in German, Ding an sich). Kant further distinguishes a negative and a positive sense of the noumenon. In the negative sense, it is a limiting concept: it marks the boundary of human knowledge, indicating that something must exist as the ground of appearances even though we cannot know it. In the positive sense, it would be an object accessible to a supposed purely intellectual intuition — a faculty that, for Kant, human beings do not possess; this positive use is therefore, strictly speaking, illegitimate. Consider, for example, a rose: we perceive its colour, its scent and its shape, but all of this belongs to the way it affects us; what the rose may be beyond that appearing remains, in principle, inaccessible. The noumenon is thus bound up with the concepts of phenomenon and thing in itself, and it lies at the heart of Kant’s transcendental idealism.
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