When all the senses are stilled, when the mind is at rest,
when the intel-
lect wavers not--then, say the wise, is reached the highest state.
This calm of the senses and the mind has been defined
as yoga. He who
attains it is freed from delusion.
Katha Upanishad 2.6.10-11
The wise man should surrender his words to his mind;
and this he should surrender to the Knowing Self;
and the Knowing Self he should surrender to the Great Self;
and that he should surrender to the Peaceful Self.
Katha Upanishad 3.13
Holding the body steady, with the three upper parts erect,
And causing the senses with the mind to enter into the heart,
A wise man with the Brahma-boat should cross over
All the fear-bringing streams.
Having repressed his breathings here in the body, and
having his movements
One should breathe through his nostrils with diminished breath.
Like that chariot yoked with vicious horses,
His mind the wise man should restrain undistractedly.
In a clean, level spot, free from pebbles, fire, and gravel,
By the sound of water and other propinquities
Favorable to thought, not offensive to the eye,
In a hidden retreat protected from the wind, one should practice yoga.
Fog, smoke, sun, fire, wind,
Fireflies, lightning, a crystal, a moon--
These are the preliminary appearances,
Which produce the manifestation of Brahman in yoga.
When the fivefold quality of yoga has been produced,
Arising from earth, water, fire, air, and space,
No sickness, old age, no death has he
Who has obtained a body made out of the fire of yoga.
Lightness, healthiness, steadiness,
Clearness of countenance and pleasantness of voice,
Sweetness of odor, and scanty excretions--
These, they say, are the first stage in the progress of yoga.
Even as a mirror stained by dust
Shines brilliantly when it has been cleansed,
So the embodied one, on seeing the nature of the Soul,
Becomes unitary, his end attained, from sorrow freed.
When with the nature of the self, as with a lamp,
A practicer of yoga beholds here the nature of Brahman,
Unborn, steadfast, from every nature free--
By knowing God, one is released from all fetters!
Svetasvatara Upanishad 2.8-15
 concentrated on the buddhi,
or the highest spiritual faculty of the soul, the individualized Atman.
This too should be submerged into the Great Self or Cosmic Mind, thereby
losing all notions of separate individuality. Finally, this Great Self,
which still knows itself, is to dissolve into the Absolute, the Peaceful
Self which is devoid of any distinction or difference whatsoever.
Compare the four states of the soul in Mandukya Upanishad, p. 834, the
four or five levels of being in Katha Upanishad 2.3.7-8, p. 93, the four
nets in Maitri Upanishad 6.28, p. 1054, and the four meditations in the
Buddha's Noble Eightfold Path, p. 170.
 The unity realized by the
adept in meditation is described in Atharva Veda 19.51.1, p. 228.
On the self-control required in meditation, see Bhagavad Gita 5.21-23,
p. 199; 6.35-36, p.733; hammapada 33-37, p. 733. Bhagavad Gita 6.10-27: