Swami Vivekananda                           

6. Where the fire is rubbed, where the air is controlled, where the Soma flows over, there a (perfect) mind is created.
               
8. Placing the body in a straight posture, with the chest, the throat and the head held erect, making the organs enter the mind, the sage crosses all the fearful currents by means of the raft of Brahman.
               
9. The man of well-regulated endeavours controls the Prâna; and when it has become quieted, breathes out through the nostrils. The persevering sage holds his mind as a charioteer holds the restive horses.
               
10. In (lonely) places as mountain caves where the floor is even, free of pebbles, fire, or sand, where there are no disturbing noises from men or waterfalls, in auspicious places helpful to the mind and pleasing to the eyes. Yoga is to be practised (mind is to be joined).

11. Like snowfall, smoke, sun, wind, fire, firefly, lightning, crystal, moon, these forms, coming before, gradually manifest the Brahman in Yoga.
               
12. When the perceptions of Yoga, arising from earth, water, light, fire, ether, have taken place, then Yoga has begun. Unto him does not come disease, nor old age, nor death, who has got a body made up of the fire of Yoga.
               
13. The first signs of entering Yoga are lightness, health, non-covetousness, clearness of complexion, a beautiful voice, an agreeable odour in the body, and scantiness of excretions.
               

14. As gold or silver, first covered with earth, and then cleaned, shines full of light, so the embodied man seeing the truth of the Atman as one, attains the goal and becomes sorrowless.

Yâjnavalkya quoted by Shankara
(In Svetâshvatara Upanishad Bhâshya.)

"After practicing the postures as desired, according to rules, then, O Gârgi, the man who has conquered the posture will practice Prânâyâma.
"Seated in an easy posture, on a (deer or tiger) skin, placed on Kusha grass, worshipping Ganapati with fruits and sweetmeats, placing the right palm on the left, holding the throat and head in the same line, the lips closed and firm, facing the east or the north, the eyes fixed on the tip of the nose, avoiding too much food or fasting, the Nâdis should be purified, without which the practice will be fruitless. Thinking of the (seed-word) "Hum," at the junction of Pingalâ and Idâ (the right and the left nostrils), the Ida should be filled with external air in twelve Mâtrâs (seconds); then the Yogi meditates on fire in the same place with the word "Rung," and while meditating thus, slowly ejects the air through the Pingala (right nostril). Again filling in through the Pingala the air should be slowly ejected through the Ida, in the same way. This should be practiced for three or four years, or three or four months, according to the directions of a Guru, in secret (alone in a room), in the early morning, at midday, in the evening, and at midnight (until) the nerves become purified. Lightness of body, clear complexion, good appetite, hearing of the Nâda, are the signs of the purification of nerves. Then should be practiced Pranayama composed of Rechaka (exhalation), Kumbhaka (retention), and Puraka (inhalation). Joining the Prâna with the Apâna is Pranayama.
"In sixteen Matras filling the body from the head to the feet, in thirty-two Matras the Prana is to be thrown out, and with sixty-four the Kurnbhaka should be made.
"There is another Pranayama in which the Kumbhaka should first be made with sixty-four Matras, then the Prana should be thrown out with sixteen, and the body next filled with sixteen Matras.
"By Pranayama impurities of the body are thrown out; by Dhâranâ the impurities of the mind; by Pratyâhâra impurities of attachment; and by Samadhi is taken off everything that hides the lordship of the Soul."

Sânkhya

BOOK III

29. By the achievement of meditation, there come to the pure one (the Purusha) all powers of nature.
               
30. Meditation is the removal of attachment.
               
31. It is perfected by the suppression of the modifications.
               
32. By Dhâranâ, posture, and performance of one's duties, it is perfected.
               
33. Restraint of the Prâna is by means of expulsion and retention.        
               
34. Posture is that which is steady and easy.

36. Also by non-attachment and practice, meditation is perfected.
               

74. By reflection on the principles of nature, and by giving them up as "not It, not It" discrimination is perfected.

BOOK IV

3. Instruction is to be repeated.
               
5. As the hawk becomes unhappy if the food is taken away from him and happy if he gives it up himself (so he who gives up everything voluntarily is happy).
               
6. As the snake is happy in giving up his old skin.
               
8. That which is not a means of liberation is not to be thought of; it becomes a cause of bondage, as in the case of Bharata.
               
9. From the association of many things there is obstruction to meditation, through passion, aversion, etc., like the shell bracelets on the virgin's hand.
               
10. It is the same even in the case of two.
               
11. The renouncers of hope are happy, like the girl Pingalâ.
               
13. Although devotion is to be given to many institutes and teachers, the essence is to be taken from them all as the bee takes the essence from many flowers.
               
14. One whose mind has become concentrated like the arrowmaker's does not get his meditation disturbed.

15. Through transgression of the original rules there is non-attainment of the goal, as in other worldly things.
               
19. By continence, reverence, and devotion to Guru, success comes after a long time (as in the case of Indra).
               
20. There is no law as to time, as in the case of Vâmadeva.
               
24. Or through association with one who has attained perfection.
               

27. Not by enjoyments is desire appeased even with sages (who have practiced Yoga for long).

BOOK V

128. The Siddhis attained by Yoga are not to be denied like recovery through medicines etc.

BOOK VI

24. Any posture which is easy and steady is an Âsana; there is no other rule.

Vyâsa-Sutras

CHAPTER IV, SECTION I

7. Worship is possible in a sitting posture.
               
8. Because of meditation.
               
9. Because the meditating (person) is compared to the immovable earth.
               
10. Also because the Smritis say so.
               
11. There is no law of place; wherever the mind is concentrated, there worship should be performed.
These several extracts give an idea of what other systems of Indian Philosophy have to say upon Yoga.