In the Critique of Pure Reason Immanuel Kant distinguished between objects as phenomena, which are objects as shaped and grasped by human sensibility and understanding, and objects as things-in-themselves or noumena, which do not appear to us in space and time and about which we can make no legitimate judgments.

The noumenon is knowledge of an object or event without the use of the senses.  Noumenon’s opposite is ‘phenomenon’ – knowledge from use of the senses.

In Ancient philosophy, the noumenal realm was equated with the world of ideas known to the philosophical mind (eg, Plato’s ‘Forms’), in contrast to the phenomenal realm, which was equated with the world of sensory reality (eg, Aristotle’s ‘Particulars’).

Modern philosophy has generally denied the possibility of knowledge independent of the senses, and Immanuel Kant gave this point of view its classical version, saying that the noumenal world may exist, but it is completely unknowable to humans. In Kantian philosophy the unknowable noumenon is often linked to the unknowable "thing per se" (Ding an sich), although how to characterize the nature of the relationship is a question yet open to some controversy.