Atma Upanishad 1-4

Next the supreme Self, the imperishable, He is to meditated on with (the help of) the Yogic steps, breath control, withdrawal (of sense organs), fixation (of mind), contemplation and concentration, He is to be inferred by the thinkers on the Self as like unto the seed of the Banyan tree or a grain of millet or a hundredth part of a split hair. (Thus) is He won and not known. He is not born, does not die, does not dry, is not wetted, not burnt, does not tremble, is not split, does not sweat. He is beyond the gunas, is spectator, is pure, partless, alone, subtle, owning naught, blemishless, immutable, devoid of sound, touch, colour, taste, smell, is indubitable, non-grasping, omnipresent. He is unthinkable and invisible. He purifies the impure, the unhallowed. He acts not. He is not subject to empirical existence.