Mundaka Upanishad 3.1

Two birds, inseparable friends, cling to the same tree. One of them eats the sweet fruit, the other looks on without eating.

On the same tree man sits grieving, immersed, bewildered by his own impotence (an-îsâ). But when he sees the other lord (îsa) contented and knows his glory, then his grief passes away.

When the seer sees the brilliant maker and lord (of the world) as the Person who has his source in Brahman, then he is wise, and shaking off good and evil, he reaches the highest oneness, free from passions;

For he is the Breath shining forth in all beings, and he who understands this becomes truly wise, not a talker only. He revels in the Self, he delights in the Self, and having performed his works (truthfulness, penance, meditation, &c.) he rests, firmly established in Brahman, the best of those who know Brahman.

By truthfulness, indeed, by penance, right knowledge, and abstinence must that Self be gained; the Self whom spotless anchorites gain is pure, and like a light within the body.

The true prevails, not the untrue; by the true the path is laid out, the way of the gods (devayânah), on which the old sages, satisfied in their desires, proceed to where there is that highest place of the True One.

That (true Brahman) shines forth grand, divine, inconceivable, smaller than small; it is far beyond what is far and yet near here, it is hidden in the cave (of the heart) among those who see it even here.

He is not apprehended by the eye, nor by speech, nor by the other senses, not by penance or good works. When a man's nature has become purified by the serene light of knowledge, then he sees him, meditating on him as without parts.

That subtle Self is to be known by thought (ketas) there where breath has entered fivefold; for every thought of men is interwoven with the senses, and when thought is purified, then the Self arises.

Whatever state a man whose nature is purified imagines, and whatever desires he desires (for himself or for others), that state he conquers and those desires he obtains. Therefore let every man who desires happiness worship the man who knows the Self.