The Western tradition uses the word "philosophy" (love of wisdom) to denote the study of the fundamental nature of reality. In the Indian tradition, we use the word "darshana" (point of view) to denote the study of existence, meaning, consciousness, and the ultimate reality. It provides us the means to the same ultimate goal, called by different names - ananda (bliss); moksha (liberation); or oneness with brahman, the Supreme Being.
What we now call as "philosophy" and "religion" is closely enmeshed in the Hindu tradition. Every action - sacred or secular - was ideally a means for progressing on the path to realising the ultimate. In Hinduism, we find elements of philosophy in a mundane manual of rituals; we also find references to rituals in a sublime work of philosophy.
For a philosopher in ancient India, it was imperative to embrace both theory and practice. One could not be a philosopher in the academic sense; s/he had to let the philosophy influence all aspects of life. Most Indian philosophers explain how their propositions are connected with the four purusharthas - desires (kama), means to fulfil the desires (artha), framework to fulfil the desires (dharma), and letting go of desires (moksha).
Indian philosophy has six classical schools and three atheistic schools. The primary distinction between the two groups is that the classical schools accepted the vedas as authority while the atheistic schools did not. It is noteworthy that the Vedas form the basis for all these schools of philosophy - while the former group built upon the wisdom of the vedas, the latter group deviated from the vedas, primarily in the practical/ritualistic aspects.
The six classical schools are often recalled in pairs:
1. Sankhya (method of reasoning, enumeration) and Yoga (union of body and mind) deal with the spiritual realm.
2. Nyaya (study of the nature and scope of knowledge) and Vaisheshika (study of the nature and meaning of existence) deal with the physical world.
3. Mimamsa (preparation for philosophical pursuits; concepts behind rituals) and Vedanta (philosophical pursuits; transcending ritual) deal with the physical world as well as the spiritual realm.
The three atheistic schools of Indian philosophy are - Jaina (Jainism), Bauddha (Buddhism) and Lokayata (materialism). While all of them reject the traditional notion of a God, both the Jaina and Bauddha traditions have certain denominations that consider Mahavira and Buddha as Gods.
Jainism is one of the earliest rationalist movements in the world. The Jaina school divides the universe into two - animate (jiva) and inanimate (ajiva). It considers the number of jivas to be infinite, identical and eternal. Ajiva is divided into five parts - time (kaala), space (akasha), matter (pudgala), motion (dharma), and stability (adharma). Reality is said to comprise of birth, sustenance, and death.
Jainism had an early theory about the existence of atoms (much like the Vaisheshika school) and speaks about identical, indivisible particles that can give rise to a variety of objects.
Jainism believes that the nature of the universe is indeterminable (syat vada) and also that reality is perceived differently by different people (anekanta vada) and one can never know the truth unless s/he becomes omniscient (kevalin).
The practical aspects of Jainism have played a defining role in the Indian culture. It was the first school of thought that emphasised on non-injury (ahimsa) in entirety. This meant a strictly vegetarian diet (which even excludes vegetables that grow under the ground) and abstinence from killing all life forms.
Buddha's teachings were passed on from disciple to disciple for many years after his death. They were collected and recorded in the tripitakas (three baskets of tradition). The Bauddha school is utterly pragmatic. It says that life is full of suffering (dukkha) and our aim is to break free from the suffering. It suggests that we can achieve peace here and now. It discards the theory of the afterlife but believes in rebirth.
Buddhism emphasises perception over speculation. It doesn't care much about things that we don't know about. It is only concerned with getting rid of suffering in this life, in this world and is wholly uninterested in empty curiosity about metaphysical aspects like creation of the universe and transmigration of the soul. It believes that everything is transient and ever-changing (kshanika vada). The ultimate goal is to seek release (nirvana) from suffering.
While there are many schools of Buddhism, the four schools often referred to (and refuted) by the philosophers of the classical schools are: Vaibhasika, Sautranika (which are from the Theravada tradition), Yogachara, Madhyamika (which are from the Mahayana tradition).
This is an ancient system of materialism propounded by Charvaka (or Brihaspati). We don't have any foundational texts of this school of philosophy. They are all lost. What we do know about the Lokayatas is from later works that have been composed to refute the system. In some ways, these seem to be a caricature of the original system with the weaknesses highlighted and few of the main tenets misrepresented.
The Lokayata school believes that human sensory perception is the only valid means of knowledge. There is nothing supernatural or spiritual. For example, they don't believe even in the idea of conscience. They abhor all kinds of rituals and do not believe in any kind of authority, let alone Vedic authority. It is a crude form of utilitarianism and reductionism. It propagates instant gratification without paying heed to the possibility of pain in the future.
The modern world has its own understanding of the world and thus its own schools of philosophy. When we look at ancient India and its conception of the darshanas, it is noteworthy to see the status given to the schools of thought that were opposed to the mainstream. The three atheistic schools were not banned or looked down upon but instead allowed to peacefully coexist. Battles were fought with words and not swords, thus allowing freedom of expression to naturally flourish.
Hiriyanna, M. Outlines of Indian Philosophy. London: George Allen and Unwin, 1932.