noumenon (nōˈmənŏnˌ) [key], in the philosophical system of Immanuel Kant, a "thing-in-itself"; it is opposed to phenomenon, the thing that appears to us. Noumena are the basic realities behind all sensory experience. According to Kant, they are not knowable because they cannot be perceived, but they must be thinkable because moral decision making and scientific investigation cannot proceed without the assumption that they exist.
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