In philosophy, a noumenon (/ˈnuːmənɒn/, /ˈnaʊ-/; from Ancient Greek: νοούμενoν; pl.: noumena) is knowledge posited as an object that exists independently of human sense. 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. 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.

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'. A rough equivalent in English would be "that which is thought", or "the object of an act of thought".

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.

Regarding the equivalent concepts in Plato, Ted Honderich writes: "Platonic Ideas and Forms are noumena, and phenomena are things displaying themselves to the senses... This dichotomy is the most characteristic feature of Plato's dualism; that noumena and the noumenal world are objects of the highest knowledge, truths, and values is Plato's principal legacy to philosophy."

As expressed in Kant's Critique of Pure Reason, human understanding is structured by "concepts of the understanding" or pure categories of understanding, found prior to experience in the mind and which make outer experiences possible as counterpart to the rational faculties of the mind.

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). Kant posited methods by which human understanding makes sense of and thus intuits phenomena that appear to the mind: the concepts of the transcendental aesthetic, as well as that of the transcendental analytic, transcendental logic and transcendental deduction. Taken together, Kant's "categories of understanding" are the principles of the human mind which necessarily are brought to bear in attempting to understand the world in which we exist (that is, to understand, or attempt to understand, "things in themselves"). 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. Kant asserts that to "transcend" a direct observation or experience is to use reason and classifications to strive to correlate with the phenomena that are observed.^{[citation needed]} Humans can make sense out of phenomena in these various ways, but in doing so can never know the "things-in-themselves", the actual objects and dynamics of the natural world in their noumenal dimension - this being the negative, correlate to phenomena and that which escapes the limits of human understanding. By Kant's Critique, our minds may attempt to correlate in useful ways, perhaps even closely accurate ways, with the structure and order of the various aspects of the universe, but cannot know these "things-in-themselves" (noumena) directly. Rather, we must infer the extent to which the human rational faculties can reach the object of "things-in-themselves" by our observations of the manifestations of those things that can be perceived via the physical senses, that is, of phenomena, and by ordering these perceptions in the mind help infer the validity of our perceptions to the rational categories used to understand them in a rational system. This rational system (transcendental analytic), being the categories of the understanding as free from empirical contingency.

According to Kant, objects of which we are cognizant via the physical senses are merely representations of unknown somethings—what Kant refers to as the transcendental object—as interpreted through the a priori or categories of the understanding. These unknown somethings are manifested within the noumenon—although we can never know how or why as our perceptions of these unknown somethings via our physical senses are bound by the limitations of the categories of the understanding and we are therefore never able to fully know the "thing-in-itself".

Many accounts of Kant's philosophy treat "noumenon" and "thing-in-itself" as synonymous, and there is textual evidence for this relationship. But 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, and other scholars also argue that they are not identical. Schopenhauer criticised Kant for changing the meaning of "noumenon". But this opinion is far from unanimous. Kant's writings show points of difference between noumena and things-in-themselves. For instance, he regards things-in-themselves as existing: