ABSTRACT
The Muktika Upanisad, a late medieval text, lists 108 Upanisads, and it classifies twenty of these as Yoga Upanisads. According to the system of Yoga elaborated in Patanjali's Yogasutras, there are two eternal principles in the world, purusa and prakrti. Early proto-Samkhya and Yoga concepts can be traced back to some of the classical Upanisads, such as the Svetasvatara Upanisad. The Hamsa Upanisad is a late Upanisad affiliated with the White Yajurveda. Hamsa is identified as the atman within the heart. The Varaha Upanisad is a fairly long Upanisad in five chapters, formally affiliated with the Black Yajurveda. The Yogatattva Upanisad is a theistic text, praising Visnu as "the supreme purusa" and "a great Yogi". The Upanisad lists several forms of Yoga, such as Mantra Yoga, Layayoga, Hatha Yoga, and Raja Yoga. The Upanisad mentions four basic Yoga postures: Siddha, Padma, Simha, and Bhadra.
