The Vedas are the wellsprings of Indian literature and philosophy. The Upanishads are like secret doctrines contained in the Cedas. Inspired by the content of the Upanishads, philosopher Badarayana wrote Brahma Sutras. Since they were unintelligible to most, they were redefined by Shankara, Ramanuja and Madhwacharya.
Shankaracharya formulated the doctrine of advaita or non-dualism — that only the ultimate principle is real and all other phenomena are ephemeral. For Shankara, the apparent reality is illusive and the only recourse to dispel this illusion is self-realisation with the help of knowledge. Ramanuja’s Shribhashya is a classic Vaishnava text. According to him, Brahmn is the cause of all apparent reality. The material world is achit or unconscious, but because of being an integral part of Vishnu, apparent reality including the human soul, can never be separated from Brahmn. The means to salvation is not knowledge but faith or love of Vishnu. Madhwacharya propounded the theory of dualism. According to him, though Brahmn is the cause of the world, he is essentially different from the individual soul.
As long as one is caught in the cycle of birth and death, it is fallacious to think that the jiva or the individual soul is identical to the Brahmn. They cannot be coupled as long as the individual soul is engaged in the pursuit of material happiness. However, once ‘jiva’ has transcended all the ordeals of worldly desires, a reunion becomes possible.