Indian philosophy is typically divided along two main lines, astika (orthodox or theistic) and nastika (unorthodox or atheistic). Buddhist, Jain, and Carvaka philosophy are unorthodox because they do not accept the authority of the Vedas. The Vedas are commonly accepted by their adherents as having originally emanated from God. Therefore in the Indian tradition, any system of thought not grounded in the Vedas, even if it includes belief in God or gods, is considered atheistic.
The astika schools, originally called sanatana dharma, are collectively referred to as Hinduism in modern times and consist of six systems of philosophy Nyaya: Vaisheshika, Yoga, Samkhya, Purva Mimamsa and Vedanta. The first four of these schools accept the authority of the Vedas, but do not derive their philosophical principles from the statements of the Vedas.
The last two schools, however, Purva Mimamsa and Vedanta, base their philosophical systems specifically on the words of the Vedas. The four Vedas, namely the Rig, Yajur, Sama and Atharva, are each divided into four parts known as Samhita, Brahmana, Aranyaka and Upanishad. The first two parts are predominantly ritualistic. The Aranyakas mark the shift from ritual to philosophy, which finds its culmination in the Upanishads. The Purva Mimamsa, lit. “the earlier deliberation,” bases its principles on the earlier (purva) parts of the Vedas, namely the Samhitas and Brahmanas. Vedanta is the study of the later parts (e.g. the Upanishads), and therefore also is called the Uttara Mimamsa, or the later deliberation.
The six astika philosophical systems, commonly referred to as Hinduism, include:
1. Sankhya. The central idea in this system is that a living being can become free from ignorance by understanding the twenty-four elements that constitute matter. (There are two types of Sankhya philosophy—one theistic, the other atheistic.)
2. Yoga. In this system, the mind is accepted as the cause of bondage and also the cause of salvation. By meditation, one should control the mind and thus transcend matter. This system was propounded by the sage Patanjali.
3. Nyaya. This is a system of logic. It states that there are sixteen knowable entities and four means of knowing. With their help one should understand the ultimate reality and attain salvation. Nyaya was propounded by Gautama Muni.
4. Vaisheshika. This system was developed by sage Kanada. He taught that there are seven padarthas or ontological entities and that understanding these leads to self-realization.
5. Purva Mimamsa. The gist of this system, taught by Jaimini, is that one attains perfection by performing sacrifices according to the Vedic injunctions.
6. Uttara Mimamsa. This system is more commonly known as Vedanta, which means “the supreme end of knowledge.” Its writings were compiled by Vedavyasa, the guru of Jaimini. It has two branches—personal and impersonal. In the former, devotion to a Personal God is the means to perfection. In the latter, one realizes oneself as the all-pervading, impersonal Absolute Truth.
Because certain of the systems complement one another, the six systems are generally paired into three groups, Sankhya and Yoga, Nyaya and Vaisheshika, and Purva Mimamsa and Vedanta. However, Vedanta is widely accepted as the apex of all six systems because it deals exclusively with the Absolute Truth and it is the only school that has maintained a consistent relevance through the modern era, though Yoga is also now popular. There are various schools of thought within Vedanta, but they can all be categorized into two divisions: impersonal and personal.
Originally the Vedas were too vast for a person to study and assimilate in a single lifetime, not to speak of discerning their conclusions. Therefore Vyasa, the compiler of the Vedas, summarized their essence in Sanskrit sutras, terse codes. Thus the Vedanta-sutras, also known as the Brahma-sutras, set forth the essence of Vedic wisdom.
Various thinkers have focused their attention on understanding and explaining the Vedanta-sutras through elaborate commentaries. These commentators fall into two general categories — Advaita-vada (impersonalism) and Dvaita-vada (personalism).
The Advaita-vada interprets the sutras to mean that the Absolute Truth is formless. Having no personal attributes, the Absolute is an eternally conscious but otherwise featureless state of existence, to which all variegated manifestations are inferior. This view held by the Advaita-vada is called the impersonal conception. The favorite slogan of the Advaita-vada is brahma satyam jagan mithya: impersonal reality is the only truth, and all else is illusory.
Dvaita-vada interprets the same sutras to reach the opposite conclusion: The Absolute Truth is a person. He has a spiritual form and many variegated attributes. The impersonal feature described above is but the brilliant light emanating from the transcendental body of this Absolute Person. This is the personal conception of the Absolute Truth.