The hymns of the Rig Veda are addressed to a huge collection of devas , or gods; however, there are several references to a single god or single principle which is the source or the totality of all other divinities and phenomenon in the universe; this is an early attempt in Hinduism to unify the many into the one. This single, originary divinity is sometimes the combination of all the gods put together, the Vishvadevas, or "Allgods." Sometimes this single divinity or principle is a concrete, active force, such as the World Maker (Vishvakarman: "All-maker"), or it's highly abstract, such as Rita, or cosmic order, an idea similar in some respects (but different in others) to the Chinese concept of the Tao or the Great Ultimate.

Upanishadic literature tended to talk about this unitary or single divinity, power, or principle to the exclusion of all other gods, so that philosophically Indian thought during the Upanishadic period moved towards many of the same conclusions as Parmenides and the Eleatic philosophers did in ancient Greece. This single, unitary divinity had several aspects and names in the Upanishads, two of the most important are Atman, "Universal Spirit," and Brahman. The word "brahman" in Sanskrit originally meant "power" and specifically referred to the power of prayer or sacrifice to bring about material change in the world (hence the word brahmin for priest); so that Brahman seems to refer to the power that brings about and changes the physical universe. In the Upanishads , Brahman is not only the principle and creator of all there is, but is also the sum totality of the universe and its phenomena.

This dual nature of the single divinity or totality of the universe, Brahman and Atman, gets worked out in the following way. Brahman can be located both in the physical, external world and also in the spiritual and inner world where it is present as Atman, "universal spirit." Now every human being has an undying soul (atman) which, because of samsara, lasts through eternity from life to life; this undying atman is a microcosm of Atman, the universal spirit, which is identical to Brahman. By understanding your true Self, by coming to know one's own undying soul, one then arrives at the knowledge of Brahman itself; the key to understanding the nature of the one unitary principle of the universe is to see one's (undying) self as identical with Brahman: "aham asmi Brahman": I am Brahman.

Here's the equation: Brahman=Atman=atman. Brahman is the totality of the universe as it is present outside of you;, Atman is the totality of the universe as it is present within you; Brahman is the totality of the world known objectively, Atman is the totality of the world known subjectively. This equation fundamentally underlies the whole of Krishna's teachings concerning dharma in the Baghavad Gita .

In the later development of Hinduism, Brahman would become one aspect of a triune god and would represent the creation aspects of that god.