The science of yoga was born in an age when mankind was more enlightened, and could easily grasp truths for which our most advanced thinkers are only groping (referring to ordinary men whose means of understanding are the clumsy tools of logic - and not the great saints). Truth is eternal. Man can perceive it; he cannot create it.
Once his perception is keen enough to behold Absolute Truth, he will partake of a reality that all share who attain the same vision. The great religions have come to man from those regions. The greatest spiritual teachers in all times have spoken from that vision. The endeavour of great teachers is to bring man back to central, eternal realities. In our age a number of such great masters have appeared. They have come with different missions, each one to stress a different aspect of Truth, each aspect sorely needed by modern man in general, or by the groups of disciples to whom they spoke in particular. As part of this present-day renaissance of ancient teachings, one particular line of great masters have devoted their lives to reestablishing the original, central teachings and practices of yoga.
The history of yoga suggests two principles that are basic to the successful practice of even the simplest asana (posture).
Firstly, the widespread enlightenment that was said to have existed at the time of yoga’s beginnings has been attributed to the proximity of our solar system to the mighty radiations of energy pouring from the centre of our galaxy. In even the darkest age, however, some souls are fully enlightened. And in even the most enlightened age some men live in a self-created darkness. The most important thing for man to remember is that he must receive enlightenment; he cannot manufacture it. A room that is painted white seems brighter because it reflects more light than will any other colour. The purpose of yoga, similarly, is to open the windows of the mind, and to awaken every cell of the body and brain to reflect and magnify the energy that comes to it from the surrounding universe. (A comparison might be drawn to modern transistor radios which, because of their efficiency, can pick up programmes where, a few years ago, nothing so small would have been able to get a sound). As you pursue your yoga practices remember that your aim must be to become spiritually completely open, to receive. Never hurry. Never strain. Feel that what you do is, in a sense, being done through you, by your willing cooperation with Divine forces.
Secondly, as the history of yoga is a long record of great yogis who brought this science back again and again to its central focus, so the practice of each individual must be directed, not toward outward appearances and display, but inward to the centre of his own being. Every outward movement must proceed from this inner centre. Every posture must be an affirmation of, and must be followed by a return to, the Divine Self within.
Excerpt from Raja Yoga by Swami Kriyananda