Summary: is not heard ; it is the thinker but is not thought of ; it is the knower but is not known; it is the substratum of all knowledge but is not an object of knowledge ; it is beyond all thought and speech.* Thus, according to the Upanishads, the Atman, or the self, is essentially unknowable, for it is the ultimate knower behind all knowledge, the ultimate seer behind all sight, and the ultimate hearer behind all hearing. The Upanishads emphasize the transcendent, ineffable nature of the self, which transcends all categories of the known and the knowable. It is the eternal, unchanging essence of consciousness beyond the reach of human comprehension.