The relationships between Atman ~ being the "Self" ~ and Brahman ~ being the "World Soul" ~ are central to the religiously inspired world view of most Hindus:- The Self which is free from sin, free from old age, from death and from grief, from hunger and thirst, which desires nothing but what it ought to desire, and imagines nothing but what it ought to imagine, that it is which we must search out, that it is which we must try to understand.
He who has searched out that Self and understands it, obtains all worlds and all desires. Let a man meditate on that (visible world) as beginning, ending, and breathing in it (the Brahman)... He is my self within the heart, smaller than a corn of rice, smaller than a corn of barley, smaller than a mustard seed, smaller than a canary seed or the kernel of a canary seed. The enlightened person sees everything in the world as his own Self, just as one views earthenware jars and pots as nothing but clay".
Atman is also a major topic of discussion in the Upanishads.
The Upanishads, written between the eighth and sixth centuries BC, are dialogues between teachers and students focusing on metaphysical questions about the nature of the universe. Many address the atman, explaining that atman is the essence of all things; it cannot be understood intellectually but can be perceived through meditation. There are six major schools of Hinduism: Nyaya, Vaisesika, Samkhya, Yoga, Mimamsa, and Vedanta.
Much like the Advaita Vedanta school, members of the Samkhya School see atman as the essence of a person and ego as the cause of personal suffering.
Unlike Advaita Vedanta, however, Samkhya holds that there are an infinite number of unique, individual atmans—one for every being in the universe.
The concepts of atman and Brahman are generally described metaphorically in the Upanishads; for example, the Chandogya Upanishad includes this passage in which Uddalaka is enlightening his son, Shvetaketu: As the rivers flowing east and west Merge in the sea and become one with it, Forgetting they were separate rivers, So do all creatures lose their separateness When they merge at last into pure Being. Nyaya scholars suggest that consciousness exists as part of the atman, and use rational arguments to support the existence of atman as an individual self or soul.
The This school of Hinduism is described as atomistic, meaning that many parts make up the whole of reality.
Knowing atman is simply understanding what atman is—but it does not lead to unification with Brahman or to eternal happiness. Unlike the other schools, it describes atman as identical with ego, or personal self.
Virtuous actions have a positive impact on one's atman, making ethics and good works particularly important in this school.