When you get into an auto, turn on the ignition, press the accelerator, and guide the forward-moving vehicle by means of the steering-wheel, you are acting on a number of premises. The same is true of yoga. Yoga is not just a mechanical practice; it presupposes the vast body of metaphysical principles known as Sanatana Dharma, the Eternal Dharma, which along with yoga originated in India and is embodied in the Upanishads, Bhagavad Gita and the Yoga Sutras (Yoga Darshan) of the sage Patanjali. Intelligent and effective practice of yoga is not possible without knowing and adhering to those principles.
Since both yoga and the philosophy on which it is based are truth in the purest form, exactly like mathematics, it is not amiss to speak of Sanatana Dharma and Sanatana Yoga–Eternal Dharma (Philosophy) and Eternal Yoga.
What, then is Sanatana Dharma? The great yogi-philosopher Shankara put it this way: “I shall tell you in half a sloka [verse] what has been written in millions of books: Brahman is real. The world is illusory. The jiva is nothing other than Brahman.” Yoga is the process by which the individual person can come to know these truths for himself as eternal realities.
Next in Living the Yoga Life: The Atman/Self