Philosophical systems in India have been grouped differently by writers over time. The prevailing Orthodox/Heterodox system classification of classification is not of remote antiquity and most of the earlier classifications do not make the difference. Nyâya and Vaiseśika were seen as one system by some sections and two distinct systems by others. Similarly, Nyâya and Yoga were seen as one system in many cases and prior to the rise of Śankara’s Advaita, Mimâmsa and Vedânta were seen as one system by some writers. These varying views gave rise to a number of different classifications.
Some of the early classifications come from the Arthaśâstra and Jain Angas. In the Arthaśâstra (3rd Century BCE), the author classified systems into two groups – Trayi (the Veda) and Anvikshiki (logic). The former consisted of a faith based system built on scriptural authority. The latter was a group of three logic based systems, consisting of Lokâyata, Sânkhya and Yoga. Some scholars have argued that Yoga in the Arthaśâstra actually means Nyâya. Another early age classification is found in the Jain Angas which enumerate Vaiseśika, Buddhism, Kapila, Lokâyata and Shastitantra. As Shastitantra and Kapila are both associated with Sânkhya, it is likely that they were distinct schools back then, possibly the two flavors of theistic Sânkhya and atheistic Sânkhya, a topic covered in some detail by Dasgupta. This classificiation in the Jain Angas appears to be quite ancient as by the time of Ishvara Krishna (2nd century CE or earlier), the Shashti Tantra was seen as a text belonging to Kapila’s school of Sânkhya.
In modern times, the most well known classification of Darśanas (philosophies) is the set of six âstika Darśanas – Sânkhya, Yoga, Nyâya, Vaiseśika, Mimâmsa and Vedânta. Lokâyata has commonly been classified as one of the Nâstika Darśanas, along with Buddhism and Jainism.
However, there have been different classifications of Darśanas by different authors over the centuries and the presently accepted scheme is relatively recent. The first known classification is by Bhâvaviveka, a Buddhist (5th Century CE). Along with Madhyamika (his school), he lists the following six – Hinayana Buddhism, Yogachara Buddhism, Vaiseśika, Sânkhya, Vedânta and Mimâmsa. The Buddhist text Manimekalai (in tamil) also from the same time, lists the following six – Lokâyata, Buddhism, Sânkhya, Nyâya, Vaiseśika and Mimâmsa.
The Jain scholar Haribhadra Suri (8th Century CE) presented.the following six fold classification – Buddhism, Nyâya, Sânkhya, Jaina, Vaiseśika and Mimâmsa. Some Jaina scholars listed Lokâyata as a seventh school. The famed Nyâya scholar Jayanta Bhatta (9th Century) concurs with Haribhadra Suri, but he replaces Vaiseśika with Lokâyata.
The number six holds a special significance in this context. There are several six-fold classifications, some pairing up schools as Sânkhya/Yoga, Mimâmsa/Vedânta, Nyâya/Vaiseśika and then listing the three Nâstika philosophies Buddhism, Jainism and Lokâyata to bring the total count to six. Notably, the Sarva-darśana-saṅ̇graha (14th Century CE) does not attempt a six fold classification and lists sixteen Darśanas starting with Lokâyata. There is no clear information on how and when the number six became important for classifications. Though there were exceptions, most classifications kept the total count to six, sometimes grouping related systems, to make it possible. The present classification of six orthodox philosophies and three heterodox philosophies appears to have taken precedence, around the seventeenth Century CE.