noun.with regard to the thought of German philosopher Immanuel Kant (1724-1804), a thing in itself as compared with a thing recognized by way of the senses and one's perceptions. Even though the noumena are the facilitators of one's experience, they can never be experienced themselves, as they lie outside of space and time, nor can they be contained by speculative logic, because the basic ideas of quantity, relation, quality, and so on, are relevant only to phenomena.