Thing in itself

In metaphysics, the noumenon (/ˈnɒuːmᵻnɒn/, from Greek: [εν]νοούμενον) is a posited object or event that exists without sense or perception. The term noumenon is generally used in contrast with or in relation to phenomenon, which refers to anything that can be apprehended by or is an object of the senses. Modern philosophy has generally been skeptical of the possibility of knowledge independent of the senses, and Immanuel Kant gave this point of view its canonical expression: that the noumenal world may exist, but it is completely unknowable through human sensation. In Kantian philosophy, the unknowable noumenon is often linked to the unknowable "thing-in-itself" (in Kant's German, Ding an sich), although how to characterize the nature of the relationship is a question yet open to some controversy.

The Greek word νοούμενoν nooúmenon, plural νοούμενα nooúmena, is the neuter middle-passive present participle of νοεῖν noeîn "to think, to mean", which in turn originates from the word νοῦς noûs, an Attic contracted form of nóos "perception, understanding, mind". A rough equivalent in English would be "something that is thought", or "the object of an act of thought".

The Oxford Companion to Philosophy proclaims "Platonic Ideas and Forms are noumena, and phenomena are things displaying themselves to the senses. [...] that noumena and the noumenal world are objects of the highest knowledge, truths, and values is Plato's principal legacy to philosophy." However, that noumena and the noumenal world were objects of the highest knowledge, truths, and values were disputed from the start, beginning with Democritus, his follower Pyrrho, founder of Pyrrhonism, and even in the Academy starting with Arcesilaus and the introduction of Academic Skepticism. In these traditions of philosophical skepticism, noumena are suspected of being delusions. Plato's allegory of the cave may be interpreted as an illustration of the noumenal/phenomenal distinction.

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