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“(pl. noumena) ‘Thing-in-itself’ contrasted with appearance or phenomenon in the philosophy of Kant. Noumena are the external source of experience but are not themselves knowable and can only be inferred from experience of phenomena. Although inaccessible to speculative reason, the noumenal world of God, freedom, and immortality is apprehended through man’s capacity for acting as a moral agent.”
— A Dictionary of Philosophy, 2nd ed., ed. Antony Flew, 251.
“By Kant’s view, humans can make sense out of phenomena in . . . various ways, but can never directly know the noumena, the “things-in-themselves”, the actual objects and dynamics of the natural world. In other words, 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 thoughts correspond with things-in-themselves by our observations of the manifestations of those things that can be sensed, that is, of phenomena.”
— “Noumenon,” Wikipedia.