The nature of the Self according to Katha upanishad is that, it is unborn, eternal, everlasting, ancient and not destroyed. There is no birth and death for the intelligent Self. It is all pervading, far, near and everywhere. It is joyful as well as joyless. It is bodiless and all pervading. It is smaller than the smallest; at the same time it is greater than the greatest. The purusa is the subtlest. It is subtler than sensory objects, senses, mind, intellect and even mahat, the manifested Hiranyagarbha and the unmanifested avyakta.
Self is hidden in all things.The Self is soundless, formless, imperishable, tasteless, odourless, eternal, constant, all pervading, subtlest, omniscient and omnipotent. Its light is greater than that of the Sun, Moon, Stars, lightning and fire. It is not seen but by its light everything shines. No mortal beings ever live by just life breath alone. One lives by the self over which the whole existence is dependent on. It is the Brahman that sustains everything and becomes the source of all. He is compared to the Peepul tree whose roots are above in the sky and the branches below.