The Upanishads

There is one Supreme Ruler, the inmost Self of all beings, who makes His one form manifold. Eternal happiness belongs to the wise, who perceive Him within themselves-not to others.

Yajur Veda, Katha Upanishad, Part Two, Chapter II, 12

There is One who is the eternal Reality among non-eternal objects, the one truly conscious Entity among conscious objects and who, though non-dual, fulfils the desires of many. Eternal peace belongs to the wise, who perceive Him within themselves-not to others.

Yajur Veda, Katha Upanishad, Part Two, Chapter II, 13

The sages realise that indescribable Supreme Joy as This is That. How can I realise It? Is It self-luminous? Does It shine brightly, or not?

Yajur Veda, Katha Upanishad, Part Two, Chapter II, 14

The sun does not shine there, nor the moon and the stars, nor these lightnings-not to speak of this fire. He shining, everything shines after Him. By His light all this is lighted.

Yajur Veda, Katha Upanishad, Part Two, Chapter II, 15