[nandm]

The trunk, or foundation, of yoga is that we are spiritual beings in human form who are here to find out that we are spiritual beings in human form! I am oversimplifying, of course, but according to the basic science of yoga, there is the soul, or transcendental self, called the atman, and there is the mind-body. Our true identity is found in merging these parts into a unified whole. This, then, is the trunk of yog from which the spreading limbs, or guiding principles, of yoga emerge and derive their strength. The science of yoga was known and practiced but not clearly documented until AD 200, when a physician-sage named Patanjali systematized and codified the science of yoga into eight limbs. The name given to this text is the Yoga sutras.

There is a lucid and very readable book on Patanjali's Yoga called How to Know God, by Christopher Isherwood and Swami Prabhavanda. This work is a "must have" classic for serious students of life!

The Yoga Sutras are the codification of yoga's principles into a written system. Sutra means "thread" (as in the Latin "suture"). Just as you might thread keys on a chain, one for your car and one for your house, in order to keep track of them, the Yoga sutras served to thread together and keep track of yoga's teachings for many centuries.