46. Posture is that which is firm and pleasant.
Now comes Asana, posture. Until you can get a firm seat you cannot practise the breathing and other exercises. Firmness of seat means that you do not feel the body at all. In the ordinary way, you will find that as soon as you sit for a few minutes all sorts of disturbances come into the body; but when you have got beyond the idea of a concrete body, you will lose all sense of the body. You will feel neither pleasure nor pain. And when you take your body up again, it will feel so rested. It is the only perfect rest that you can give to the body. When you have succeeded in conquering the body and keeping it firm, your practice will remain firm, but while you are disturbed by the body, your nerves become disturbed, and you cannot concentrate the mind.
47. By lessening the natural tendency (for restlessness) and meditating on the unlimited, posture becomes firm and pleasant.
We can make the seat firm by thinking of the infinite. We cannot think of the Absolute Infinite, but we can think of the infinite sky.
ततो द्वन्द्वानभिघातः ॥४८॥
48. Seat being conquered, the dualities do not obstruct.
The dualities, good and bad, heat and cold, and all the pairs of opposites, will not then disturb you.
- Swami Vivekananda
The Complete Works of Swami Vivekananda/Volume 1/Raja-Yoga/Patanjali's Yoga Aphorisms - Concentration: Its Practice