Interspersed throughout the hymns collected in the Rig Veda are references to a single god or single principle which is the source or the totality of all other divinities and phenomenon in the universe. These originary divinities can be concrete, such as the World Maker (Vishvakarman), or highly abstract, such as Rita, or cosmic order, an idea similar 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 most other gods, so that philosophically Indian thought during the Vedantic period moved towards the One in the dichotomy of the one and the many, approaching in many instances some of the same conclusions Parmenides and the Eleatic philosophers did in ancient Greece.

This single, unitary divinity had several aspects and names in the Upanishads, one of the most important of which is Atman, a word that originally meant "breath" or "soul" or "vital principle" (as the word "Atmen" does in German). As a cosmological principle or deity, Atman seems to be something like "universal soul" or "universal spirit." In the Brihad-Aranyaka Upanishad , Atman is explicitly called a Person that created the universe by first splitting himself into male and female halves. In the Chandogya Upanishad, this single god is called Brahman, and is "the One without a second"; this Brahman is not only the principle and creator of all there is, but is also fully present within each individual.

 This dual conception, 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. By understanding yourself, by coming to know one's own soul, one then arrives at the knowledge of Atman 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 that principle: "tat svam asi": That (Atman) is what you are, Svetaketu. (Chandogya Upanishad VI.8.4ff.)

 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.