Famously, Kant made a distinction between the things-in-themselves (noumena) that lie beyond the reach and realm of human thinking and the things (phenomena) that we encounter (intuit) in everyday experiences as they appear in consciousness.
Phenomena are the many kinds of sensory things that we come to know in the way they appear through our perceptions of the world around us. But all these things (whether natural or human-made) have a human face in that we can only make sense of them with our human cognitive faculties. What the things themselves “really” look like is beyond our human grasp. Kant termed these unknowables “noumena,” even though the meaning of noumenon derives etymologically from the Greek noumenon, meaning “thought of,” and nous, meaning “mind.”
Kant’s noumenon is sometimes synonymous with “das Ding an sich,” the “thing-in-itself” or otherwise noumenon seems to refer to the existence of the things-in- themselves. But, for Kant, the noumena cannot be directly known by the mind through human categories. Noumena can only be imagined. In this sense, noumena are the unknowable roots of things and the meaning of things.
Selected Readings:
Kant, I. (1999). Critique of Pure Reason. (P. Guyer and A. W. Wood, transl.) Cambridge, UK: Cambridge University Press.