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 νόος, nóos, 'perception, understanding, mind'.
The Indian Vedānta philosophy (specifically Advaita), the roots of which go back to the Vedic period, talks of the ātman (self) in similar terms as the noumenon.
[6] Regarding the equivalent concepts in Plato, Ted Honderich writes: "Platonic Ideas and Forms are noumena, and phenomena are things displaying themselves to the senses...
[8][9] By Kant's account, when one employs a concept to describe or categorize noumena (the objects of inquiry, investigation or analysis of the workings of the world), one is also employing a way of describing or categorizing phenomena (the observable manifestations of those objects of inquiry, investigation or analysis).
In each instance the word "transcendental" refers to the process that the human mind must exercise to understand or grasp the form of, and order among, phenomena.
[15] Many accounts of Kant's philosophy treat "noumenon" and "thing-in-itself" as synonymous, and there is textual evidence for this relationship.
[16] However, Stephen Palmquist holds that "noumenon" and "thing-in-itself" are only loosely synonymous, inasmuch as they represent the same concept viewed from two different perspectives,[17][18] and other scholars also argue that they are not identical.
But Stephen Palmquist explains that this is part of Kant's definition of the term, to the extent that anyone who claims to have found a way of making the thing-in-itself knowable must be adopting a non-Kantian position.
[26] The positive noumena, if they existed, would be immaterial entities that can only be apprehended by a special, non-sensory faculty: "intellectual intuition" (nicht sinnliche Anschauung).
But in so doing it at the same time sets limits to itself, recognising that it cannot know these noumena through any of the categories, and that it must therefore think them only under the title of an unknown something.
Though the term noumenon did not come into common usage until Kant, the idea that undergirds it, that matter has an absolute existence which causes it to emanate certain phenomena, had historically been subjected to criticism.
George Berkeley, who pre-dated Kant, asserted that matter, independent of an observant mind, is metaphysically impossible.
He explained in his "Critique of the Kantian philosophy", which first appeared as an appendix to The World as Will and Representation: The difference between abstract and intuitive cognition, which Kant entirely overlooks, was the very one that ancient philosophers indicated as φαινόμενα [phainomena] and νοούμενα [nooumena]; the opposition and incommensurability between these terms proved very productive in the philosophemes of the Eleatics, in Plato's doctrine of Ideas, in the dialectic of the Megarics, and later in the scholastics, in the conflict between nominalism and realism.
13) of Sextus Empiricus to demonstrate the original distinction between phenomenon and noumenon according to ancient philosophers: νοούμενα φαινομένοις ἀντετίθη Ἀναξαγόρας ('Anaxagoras opposed what is thought to what appears.')