In Indian philosophy, three schools of thought are commonly referred to as Nastikas namely Jainism, Buddhism and Carvaka for rejecting the doctrine of Vedas. Nastika refers to the non-belief of Vedas rather than non-belief of God. However, all these schools also rejected a notion of creationist god and so the word Nastik became strongly associated with them. Carvaka, an atheistic school of Indian philosophy, traces its origins to 600 BCE. It was a hedonistic school of thought, advocating that there is no afterlife. Carvaka philosophy appears to have died out some time after 1400 CE. Buddhism and Jainism also have their origins before 300 BCE but are opposite of Carvaka because they are not hedonistic. There are just some shifts and turns in the classical Indian concept of reason in philosophy. Recognising reason is one of the main instruments of all philosophers, as the rational principles are said to drive classical Indian philosophy.
Veda signifies wisdom or knowledge or it can be termed as the sacred knowledge, holy learning, and the scriptures of the Hindus. They are compiled in complex metres and filled with various sophisticated plays on the sounds of words. They are compiled in a language (Sanskrit), which is filled with synonyms indicating a long and rich development. Above all it has an entire mysticism of sound, mantra and the Divine Word. Vedic texts are often categorised into four classes: the Samhitas, Brahmanas, Aranyakas, and Upanishads.
The Samhitas are collections of hymns, mantras, and chants. There are four Samhitas: the Rig-Veda, Sama Veda, Yajur Veda, and Atharva Veda.
Firstly, the oldest, the Rig Veda, is a collection of 1028 Vedic Sanskrit hymns dedicated to Rigvedic deities. The Rigveda was composed roughly between 1700-1100 BCE (the early Vedic period) in the Punjab (Sapta Sindhu) region of the Indian subcontinent. It was preserved in India over centuries by oral tradition alone and was probably not put in writing until Late Antiquity or even the early Middle Ages.
Secondly, the Yajur Veda or the 'Veda of sacrificial prayers' also contains verses, largely borrowed from the Rig Veda. This Veda was compiled to apply to the entire sacrificial rite, not merely the Soma offering. There are two recensions of this Veda known as the 'Black' and 'White' Yajur Veda. The White Yajur Veda contains only the verses and sayings necessary for the sacrifice, while explanations exist in a separate work; the Black incorporates explanations and directions in the work itself, often immediately following the verses.
Thirdly, the Sama Veda or "Veda of chants" or Knowledge of melodies, consists of 1549 stanzas, taken entirely (except 78) from the Rig-Veda. The name of this Veda is from the Sanskrit word saman which means a metrical hymn or song of praise. Including repetitions of the Rig Veda verses, there are a total of 1875 verses numbered in the Sama-Veda recensions published by Griffith.
Fourthly, the Atharva Veda or "Knowledge of the Fire Priests" has 760 hymns, and about one-sixth of the hymns are in common with the Rig-Veda. An atharvan was a priest who worshipped fire and Soma. It was compiled around 900 BCE, although some of its material may go back to the time of the Rig Veda. Some parts of this Veda are considered to be much older than the Rig Veda too, but there is no evidence of the same. The Atharvana-Veda is preserved in two editions, the Paippalada and Saunaka. It consists chiefly of spells and incantations, concerned with protection against demons and disaster, spells for the healing of diseases, for long life, etc.
The Brahmanas are prose texts with prescriptions for carrying out sacrificial rituals. They also carry comments on the meaning of the rituals. Each of the Brahmanas is associated with one of the Samhitas. The Brahmanas may either form separate texts or can be partly integrated into the text of the Samhitas. They are part of the Hindu Shruti and are composed in Vedic Sanskrit. They are essentially commentaries of the Vedas, explaining Vedic ritual. The earliest Brahmanas may have been written several centuries earlier, contemporary to the Krishna Yajur Veda commentary prose, but they have only survived in fragments.
The Aranyakas or forest books are the concluding part of the Brahmanas and the Hindu Sruti, and they contain further interpretations of rituals and other materials. This contrasts with the Grhya Sutras, treatises intended for domestic life. The Aranyakas discuss philosophy and sacrifice. They are believed to have originated with the various mystical ascetic groups that developed in post-Vedic India.
The Upanishads are theological and philosophical works. They are mystic or spiritual interpretations of the Vedas, and are considered their putative end and essence, and thus known as Vedanta (the end of the Vedas). The Upanishads were composed over several centuries. The oldest, such as the Brihadaranyaka, also being the longest, and Chandogya Upanishads, have been dated to around the eighth century BC. The philosophical edifice of Indian religion viz., Hinduism, Jainism and Buddhism is built on the foundation laid by the Upanishads. The Sanskrit term Upanishad originates from upa- (near), ni- (down) and sad (to sit), that is referring to sitting down near a spiritual teacher (guru) in order to receive instruction in the Guru-shishya tradition. The term thus emphasises the esoteric nature of the texts, not intended for public teaching, but restricted to the confidentiality of personal instruction. According to tradition, there were over two hundred Upanishads, but the philosopher and commentator Shankara only composed commentaries to eleven of them. The Muktika Upanishad lists 108 Upanishads. In 1656, at the order of Dara Shikoh, the Upanishads were translated from Sanskrit into Persian. From 1802 to 1804 Abraham-Hyacinth Anquetil Du Perron published a Latin translation (2 volumes) from the Persian of the Oupnek'hat or Upanishada. It is an interesting mixture of Latin, Greek, Persian, Arabic, and Sanskrit.
Philosophy of Upanishads
The Upanishads have a universal feel that has led to the explanation of One Brahman and the inner Atman (Self). The Upanishads are summed up in one phrase - "Tat Tvam Asi", (Thou Art That) by the Advaita Vedanta and they believe that in the end, the ultimate, formless, inconceivable Brahman is the same as the soul, Atman. One only has to realise it through discrimination. The Upanishads also contain the first and most definitive explications of aum or Om as the divine word, the cosmic vibration that underlies all existence and contains multiple trinities of being and principles subsumed into its one self.
Buddhist and Jain Atomic Theories
Buddhist and Jain scholars, as well as later Hindu scholars offered their own approaches to scientific reasoning. Both the Jains and the Buddhists correctly speculated that a potential for the desired effect must also be present in the cause or causal agent. (For instance, only a mango seed could produce a mango tree because only the mango seed incorporated the potential of developing into a mango tree.) Like the Vaisheshikas, the Jainas perceived atoms as infinitely small. But the Jainas went a step further by positing that the union of atoms required opposite qualities in the combining atoms - as is true in the case of electrovalent bonding.