In the sight of true wisdom, there is no difference between the creator and the created. Even physical science has come to recognize that cause and effect are but two aspects of one manifestation of energy. He who fails to see this, being engrossed in the visible only, goes from death to death; because he clings to external forms which are perishable. Only the essence which dwells within is unchangeable and imperishable. This knowledge of the oneness of visible and invisible, however, cannot be acquired through sense-perception. It can only be attained by the purified mind.
XII
The Purusha (Self), of the size of a thumb, resides in the middle of the body as the lord of the past and the future, (he who knows Him) fears no more. This verily is That.
The seat of the Purusha is said to be the heart, hence It "resides in the middle of the body." Although It is limitless and all-pervading, yet in relation to Its abiding-place It is represented as limited in extension, "the size of a thumb." This refers really to the heart, which in shape may be likened to a thumb. Its light is everywhere, yet we see it focused in a lamp and believe it to be there only; similarly, although the life-current flows everywhere in the body, the heart is regarded as peculiarly its seat.
XIII
That Purusha, of the size of a thumb, is like a light without smoke, lord of the past and the future. He is the same today and tomorrow. This verily is That.
In this verse the teacher defines the effulgent nature of the Soul, whose light is pure like a flame without smoke. He also answers the question put by Nachiketas as to what happens after death, by declaring that no real change takes place, because the Soul is ever the same.
XIV
As rain water, (falling) on the mountain top, runs down over the rocks on all sides; similarly, he who sees difference (between visible forms) runs after them in various directions.