In philosophy, a noumenon (/ˈnuːmənɒn/, /ˈnaʊ-/; from Greek: νοούμενoν; pl.: noumena) is knowledge[1] posited as an object that exists independently of human sense.[2] The term noumenon is generally used in contrast with, or in relation to, the term phenomenon, which refers to any object of the senses. Immanuel Kant first developed the notion of the noumenon as part of his transcendental idealism, suggesting that while we know the noumenal world to exist because human sensibility is merely receptive, it is not itself sensible and must therefore remain otherwise unknowable to us.[3] In Kantian philosophy, the noumenon is often associated with the unknowable "thing-in-itself" (German: Ding an sich). However, the nature of the relationship between the two is not made explicit in Kant's work, and remains a subject of debate among Kant scholars as a result.
1. intellectual conception of a thing as it is in itself, not as it is known through perception; 2. The of-itself-unknown and unknowable rational object, or thing-in-itself, which is distinguished from the phenomenon through which it is apprehended by the physical senses, and by which it is interpreted and understood; – so used in the philosophy of Kant and his followers.