Now, all this universe was then undifferentiated. It became differentiated by name and form: it was known by such and such a name, and such and such a form. Thus to this day this universe is differentiated by name and form; so it is said. He has such aname and such a form. This Self has entered into these bodies up to the very tips of the nails, as a razor lies hidden in its case, or as fire, which sustains the world, lies hidden in its source. People do not see the Self, for when viewed in parts It is incomplete: when breathing, It is called the vital breath (prana); when speaking, the organ of speech; when seeing, the eye; when hearing, the ear; when thinking, the mind.

Yajur Veda, Brihadaranyaka Upanishad I, IV-The Creation and Its Cause, 7

These are merely Its names according to Its functions. He who meditates on one or another of Its aspects does not know, for It is then incomplete: the Self is separated from Its totality by being associated with a single characteristic. The Self alone is to be meditated upon, for in It all these become unified. Of all these, this Self alone should be known, for one knows all these through It, just as one may find an animal which is lost through its footprints. He who thus knows the Self obtains fame and association with dear ones.

Yajur Veda, Brihadaranyaka Upanishad I, IV-The Creation and Its Cause, 7 (CONT)

This Self is dearer than a son, dearer than wealth, dearer than everything else, because It is innermost. If one holding the Self dear were to say to a person who speaks of anything other than the Self as dear, that he, the latter, will lose what he holds dear-and the former is certainly competent to do so-it will indeed come true. One should meditate upon the Self alone as dear. He who meditates upon the Self alone as dear-what he holds dear will not perish.

Yajur Veda, Brihadaranyaka Upanishad I, IV-The Creation and Its Cause, 8

They say: Since men think that by the Knowledge of Brahman they become all, what, pray, was it that Brahman knew by which It became all?

Yajur Veda, Brihadaranyaka Upanishad I, IV-The Creation and Its Cause, 9

This self was indeed Brahman in the beginning. It knew itself only as I am Brahman. Therefore it became all. And whoever among the gods had this enlightenment, also became That Brahman. It is the same with the seers (rishis), the same with men. The seer Vamadeva, having realized this self as That, came to know: I was Manu and the sun. And to this day, whoever in a like manner knows the self as I am Brahman, becomes all this universe. Even the gods cannot prevent his becoming this, for he has become their Self.

Yajur Veda, Brihadaranyaka Upanishad I, IV-The Creation and Its Cause, 10