 In both cases, states David Lorenzen, they practice yoga and their principal god tends to be Nirguna, that is a god that is without form and semi-monistic, influenced in the medieval era by the Advaita Vedanta school of Hinduism, Madhyamaka school of Buddhism, as well as Tantra and Yogic practices.The Yoga-Bhashya (400 CE), the oldest extant commentary on the Yoga-Sutra offers the following fourfold classification of yogis