That which exists, as distinguished from that which merely seems to exist, the latter being a phenomenon. The noumenon is a bit difficult to locate; it can be apprehended only be a process of reasoning -- which is a phenomenon. Nevertheless, the discovery and exposition of noumena offer a rich field for what Lewes calls "the endless variety and excitement of philosophic thought." Hurrah (therefore) for the noumenon!
The of itself unknown and unknowable rational object, or thing in itself, which is distinguished from the phenomenon through which it is apprehended by the senses, and by which it is interpreted and understood; -- so used in the philosophy of Kant and his followers.
in Kant, the ultimate reality, or “Thing-in-Itself,” which can be conceived by thought, but cannot be perceived in experience.