can be seen to contain the intuition that there must be a single principle behind all phenomena: 'That one' (tad ekam), self-sufficient, to which distinctions cannot be applied. It is with the Upanishads, reckoned to be written in the first millennia (coeval with the ritualistic Brahmanas), that the Vedic emphasis on ritual was challenged. The Upanishads can be seen as the expression of new sources of power in India. Also, separate from the Upanishadic tradition were bands of wandering ascetics called Vadins whose largely nontheistic notions rejected the notion that religious knowledge was the property of the Brahmins. Many of these were shramanas, who represented a non-Vedic tradition rooted in India's pre-Aryan history. The emphasis of the Upanishads turned to knowledge, specifically the ultimate identity of all phenomena. This is expressed in the notion of Brahman, the key idea of the Upanishads, and much later philosophizing has been taken up with deciding whether Brahman is personal or impersonal. The understanding of the nature of Brahman as impersonal is based in the definition of it as 'ekam eva advitiyam' (Chandogya Upanishad 6.2.1) – it is one without a second and to which no substantive predicates can be attached. Further, both the Chandogya and Brihadaranyaka Upanishads assert that the individual atman and the impersonal Brahman are one. The mahāvākya statement Tat Tvam Asi, found in the Chandogya Upanishad, can be taken to indicate this unity. The latter Upanishad uses the negative term Neti neti to 'describe' the divine. Classical Samkhya, Mimamsa, early Vaisheshika and early Nyaya schools of Hinduism do not accept the notion of an omnipotent creator God at all. While the Sankhya and Mimamsa schools no longer have significant followings in India, they are both influential in the development of later schools of philosophy. The Yoga of Patanjali is the school that probably owes most to the Samkhya thought. This school is dualistic, in the sense that there is a division between 'spirit' (Sanskrit: purusha) and 'nature' (Sanskrit: prakṛti). It holds Samadhi or 'concentrative union' as its ultimate goal and it does not consider God's existence as either essential or necessary to achieving this. The Bhagavad Gita, contains passages that bear a monistic reading and others that bear a theistic reading. Generally, the book as a whole has been interpreted by some who see it as containing a primarily nontheistic message, and by others who stress its theistic message. These