A series of physical and mental disciplines, Yoga is a meditative practice believed to have been originated in India. Vedic Literature contains references to ascetics, as early as 900 to 500 BC. Some of the seals discovered from the Indus Valley Civilization sites have figures in postures resembling the ones in Yoga. According to the Katha Upanishad, Yoga refers to the control of the senses and mental equilibrium. The Upanishads, the Bhagavad-Gita, the Mahabharata and the Yoga Sutras of Patanjali are the most important textual sources about the concept of Yoga.