Yoga dates back thousands of years. Yoga as we know it today stems primarily from the writings of Patanjali. Patanjali was one of the first to write down an ancient oral tradition, in his Yoga Sutras (the rules of Yoga), sometime between 800 B.C. and 300 A.D. (though some claim it is much older). Divided into four short books, they contain a profound philosophy that continues to have significant influence today. There are many branches of Yoga now, but the type that was brought to the West, specifically to the United States by Paramahansa Yogananda in the 1920s (Yogananda, 1946), traces its roots directly to the Yoga of Patanjali.