Their speculations saw philosophy as something to be lived rather than simply as a vehicle for understanding or for social reform. The historical development of the schools is difficult to construct because Indian intellectuals were not particularly concerned with chronology, consequently records have been lost or were never kept.
Originally most of the schools of Hindu philosophy were nontheistic, or naturalistic, meaning they did not use stories or beliefs about the gods, goddesses, theistic tendencies into a system.
He taught that the soul (atman) is an aspect of the impersonal Absolute (Brahman) from which everything in the cosmos has come. The result is that the world is an illusion (maya) that tricks people into believing that the world is real. He taught that by means of knowledge obtained by identification with the Absolute, the soul might find release.
Shankara’s argument is nondualistic because he claims that ultimate reality (Brahman) and temporal reality are of the same essence. He opined that moksha (liberation) arises from the knowledge that Brahman and atman are one.
Shankara’s system is called Advaita (nondualism) Vedanta. Its implications for Hinduism were great. The inferences that arise from his nondualism are that the world is an illusion (maya). Furthermore, the practice of bhakti is devotion to an illusion.
For those who achieve the liberation of understanding from the Advaita system the ultimate implication is that there is only one Brahman and all else including dharma, gods, rituals, scripture, and devotional practices are illusions.
Later Vedanta philosophers rejected his radical nondualism. Ramanuja (c. 1017–1137 c.e.) was a member of the Vedanta tradition who wrote commentaries that moved devotion to a mode or avatar of Brahman back to the center of spiritual belief and practice.
His system is called Vaishnavites (qualified nondualism). This system allowed for worship of Vishnu. In the 1200s c.e. the Vaishnavite theologian Madhva taught dualism in the Davait (dualist) Vedanta school. A little earlier Ramanuja (1100s c.e.) took a middle qualified nondualistic position between Madhva and Shankara.
This meant that there was a real difference between the Brahman and the individual self that worshipped. This theology aided the development of bhakti movements in south India. It allowed for a tension between identity with the divine power (abheda) and individuality (bheda) to created bhedaabheda.
The Samkhya ("knowledge" or "wisdom") school taught "evolutionary dualism". It is probably the oldest of the Hindu philosophical systems. It is believed by some to have been founded by Kapila after 100 b.c.e. References in the Svetasvatara Upanishad and the Bhagavad Gita are considered to be references to the philosophy in its preliterate form. One of its important ideas was prakrti (matter).
Another important idea was purusha (consciousness). Both prakrti and purusha are words in the Mahabharata, suggesting that these ideas are at least as old as the Mahabharata. The opposition of prakrti and purusha was basic. Individual souls were infinite and discrete, so salvation occurred when the soul recovered its original purity, completely purged from matter.
The Samkhya school taught that prakrti is composed of three gunas ("strands" or "ropes"). The sattva ("reality" or "illumination") rope is the psychological rope that produces happiness. The raja ("foulness" or "corrupt activity") rope leads to pain.
The tama ("darkness" or "unilluminated") rope leads to darkness of mind or ignorance. The Yoga (disciplined meditation) school of philosophy is usually paired with the Samkhya school. It developed and practiced the disciplines necessary to achieve liberation from karma in accordance with Samkhya philosophy.
The yogi (practitioner of yoga) applying the Samkhya metaphysics used ascetic meditation disciplines and a strict moral code to purge himself or herself of prakrti. Eventually, the Samkhya, Yoga, and Vedanta schools adapted their philosophy so that it served as a base for their theistic system.