Noumenon (of Greek νοούμενον, "What is thought") refers to an object of human research, understanding or knowledge. Objects or events are meant to be thought of, whether the object of an act of thinking is independent of the senses. The opposite of noumenon is phenomenon. In ancient philosophy, the noumenal world was equated with the world of ideas that could only be known by the philosophical spirit. In contrast, the world of phenomena, which was equated with the world of sensual reality, was known by the illiterate spirit. Modern philosophy generally denies the possibility of knowledge that is independent of the senses. Immanuel Kant gave the classical version of this view, which states that the noumenal world can exist, but is completely unthinkable to humankind. In Kantian philosophy, the unknown noumenon is sometimes linked to the unkind "Ding an sich". Many contributions to Kant's philosophy treat "noumenon" and "thing in itself" as synonymous, and there is textual evidence for this relationship. However, Stephen Palmquist states that "noumenon" and "thing in itself" are merely loose synonymous, insofar as they represent the same concept from two different perspectives.

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