From the Taittirïya Upanishad

In the beginning this universe was not.
There was just pure potential, from which was then born Being.
And from Being was born the Self, which is known as perfect.
Truly, that perfect Self is the essence of existence.
Truly, in tasting the essence one rejoices in bliss.
Indeed, who could breathe, who could live, were there not this all-pervading bliss?
Truly, it is this essence that bestows bliss.
Truly, when a person discovers a foundation of fearlessness in the Self, in that which is invisible,
formless, unlimited, and self-sufficient, then has he found true fearlessness.
If, however, he makes in this unity even the smallest gap, then fear is born.
To all whose self is small, the form of the formless brings fear.

The Taittirïya Upanishad is one of a school of Vedic philosophy that outlines the five sheaths (kosas) of the self in order to know – intimately, intellectually, experientially – the supreme self (paramatma-jnana).