Yoga is one of the six classical schools (Darshanas) of indian philosophy that consist of a series of mental and physical exercises such as Yama, Niyama, Asanas, Pranayama, Pratyahara, Kriyas, meditation and/or asceticism. Some of these ruminant forms of yoga put emphasis on mental concentration, others more on physical exercises, positions and breathing exercises (Pranayama), some set priorities on asceticism. The philosophical background of yoga were primarily verbalized by Patanjali in Yoga-Sutra. But also the Bhagavad Gita and Upanishaden talk about yoga. yoga has been documented since at least 700 B.C.