The knower of Truth says, "I know It not," because he realizes the unbounded, infinite nature of the Supreme. "Thou art this (the visible), Thou art That (the invisible), and Thou art all that is beyond," he declares. The ordinary idea of knowledge is that which is based on sense preceptions; but the knowledge of an illumined Sage is not confined to his senses. He has all the knowledge that comes from the senses and all that comes from Spirit.
The special purpose of this Upanishad is to give us the knowledge of the Real, that we may not come under the dominion of the ego by identifying ourselves with our body, mind and senses. Mortals become mortals because they fall under the sway of ego and depend on their own limited physical and mental strength. The lesson of the parable of the Devas and Brahman is that there is no real power, no real doer except G.o.d. He is the eye of the eye, the ear of the ear; and eyes, ears, and all our faculties have no power independent of Him. When we thus realize Him as the underlying Reality of our being, we transcend death and become immortal.
OM! PEACE! PEACE! PEACE!