This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.5 India License.
1. From this previous post, I have argued that there is no term for "religion" in Indian world-view. One can say there was no religion before Islam came.
2. From this another previous post, we have seen that there are dharma-shastras (law-books dealing with societal, judicial, administrative matters etc) , Artha-shaastras (dealing with power, economics, politics, warfare etc), Kaamshastras (dealing with creative faculties of mind - finearts and performing arts), Moksha-shaastras (philosophical schools dealing with nirvana/moksha) in Indian "world-view" and no "one single Book" by "One single author" to guide people in all aspects of life.
Continuing the line of discussion further, we find out that there were many schools of thought, ideologies, philosophies which were constantly arguing and debating with each other. Every ideology (religion in modern terms) was called as "mat (मत)" by our seers.
Buddhism was called Bauddha-mat (बौद्धमत) - Opinion of Buddhaa
Jainism was called Jaaina-mat -(जैनमत) - Opinion of Jinendra
Vaishnavism was Vaishnava-mat - (वैष्णवमत)- Opinion of vaishnavas
Shaivism was called Shaiva-mat - (शैवमत) - Opinion of Shaivas
Sikh school of thought is called Guru-mat -(गुरुमत) - Opinion of Guru
So on and so forth for all the ideological schools....
Thus everything was "an opinion" onlee. Although all the philosophies claimed to be stating complete truth, all accepted and understood each-other's position and agreed to disagree without resorting to physical violence.
From this, what we follow is that in Indian system, Conversion == change of opinion. It is as simple as that. If I start finding "opinion of Gurus" logical than others in "given space and time" I will change my opinion and become a Sikh. The classical word used for "conversion" in Indian literature is Mataantar (मतांतर).
The Argumentative Indians:
The Vaad-vivaad (argumentative debate) tradition is age-old in India. The proponents of each schools used to go to other schools and hold debate sessions. There they put-forth the opinions in public forums and argued with each other (usually for days and weeks).
There are rules of acquiring and displaying knowledge in Indian system. That system is called "Nyaaya (न्याय)". In this, there is categorization of "acceptable valid proof" (Pramaanam प्रमाणं). In that list of acceptable valid proofs, there were certain schools who placed "sentence in Vedas (वेदवाक्यं)" at first position (eg. Mimaamsa मीमांसा). Others place it far below in the list (eg. Saamkhya-Yoga-Vedanta-Vaisheshika). Others do not place it at all (eg. Bauddha, Jaina, Charvaka).
Those people in whose list, "sentence in Veda" finds a mention anywhere (even if bottommost position), those people are called "aastikas". Aastika comes from "Asti (अस्ति) - Yes, it is".
Those people who would cite a reference from Vedas but not consider it as a "Pramaanam" were Naastikaas. "Naa asti (ना अस्ति) - No, it isn't".
The syntax of a typical debate is something like
Statement X - Sarvam Khalu Idam Brahma - सर्वं खलु इदं ब्रह्म (Everything that "is" is parabrahman)..
Debater 1 (aastika) agrees.
Moderator and/or the other party asks," Ko Pramaanam - को प्रमाणं?" (What is the proof?)
Debater 1 - Cites a verse from some upanishada and says "Iti pramaanam - इति प्रमाणं" (here is the proof).
Debater 2 (if he disagrees) - Naasti naasti - नास्ति नास्ति (no it isn't, no it isn't).
Debater 3 (if there and agrees) - Aasti aasti - अस्ति अस्ति (yes it is, yes it is).
Thus Debater 1 and 3 are Aastikas (Yes-sayers) and Debater 2 is Naastika (No-sayer).
Now, consider a situation in some other time and place when Debater 1 and 3 meet (lets assume 1 is mimaamsak and 3 is Vedantist). They have debate going on for days on whether Veda-vaakyam (Statement in Vedas) should be given the "numero uno" priority as proof or whether it should be one of the subsidiary supporting proofs. The famous example of this kind of debate is the one between Adi Shankara and Mandana-Mishra. Adi Shankara gives "direct experience" more preference than abidance to scriptures. Here both are "aastikas" and yet oppose each other with almost equal vehemence.
The aastika schools of Saamkhya and Yoga are ideologically close to Bauddha school of thought compared to distance between Vedanta and Bauddha. All these schools of vaishnav, shaiv, shaakta, gaanapatya, Bhakti fall under a Sub-class of Vedanta known as "Dwaitmat (द्वैतमत). There are similar vehement arguments between dualists and non-dualists (Advaita-mat).
Like pH scale if we make a scale from 0 to 10 with adherence/acceptance/influence of principles of Vedas as measuring factor with 0 being least adherence/acceptance/influence, we will find most of the naastika schools from 0 to 2-3. Because there is always some penetration of Vedic philosophy in every indian philosophy (including Islamic Sufism). The position on scale is determined by "to what extent do they allow this penetration/influence". Mimaamsak will fall on 10 (extreme adherence to vedic rituals and philosophies). Others will fall at different places.
This was just to explain the origin of terms like "Aastika" and "Naastika". There is nothing negative about any of them.
From point of view of a Mimaamsaka, Samkhya and Yogis are Nastikas. from his (mimansak's) PoV, samkhya are "less Naastika" than Bauddha or Jaina. From point of view of Charvaka (sitting on 0), everybody else are aastikas because everybody have accepted some penetration of vedic philosophy to an extent greater than Charvakas. Similarly vice-versa.
The system is furthermore complex because every individual is free to make and propound his own path (यतो मत, ततो पथ - As many opinions, so many paths). So every individual calibrates his position on the scale and relative to his position, others are Aastikas and Naastikas. The Followers of Ramkrishna mission (advaitists) and ISKCON (Achintya Bhedaabheda Dvaitists) have had similar arguments. Similarly J Krishnamurthy, Rajneesh and other modern philosophers.
I have been labelled as a Naastika by many vedantists and Bhakti-maargi people in few forums. Some Buddhists label me as Aastika. Likewise I call them by these relative degrees just to know where I stand. This is what it is all about - Recalibration of Self.
Deracination: The loss of this understanding of Aastikas and Naastikas and their misinterpretation as "Theists" and "Atheists" respectively, has led to several instances of misunderstandings of the literature. The belief in god (as implied by the word theism) has nothing to do with "Aastika" and "Naastika" philosophies. Out of 6 Aastika philosophies, 5 are atheists (they do not believe in existence of "Ishwara"). The term used for them in India is "Nirishwar-vaadi (निरीश्वरवादी)".
Categorization of few of the popular Indian religious philosophies to avoid further misinterpretations and confusions:
1. Aastika Atheist philosophies: Saamkhya, Nyaya, Vaisheshika, Purva Mimamsa, Yoga
2. Aastika Theist philosophies: Vedanta (with all its sub-categories)
3. Naastika Atheist philosophies: Buddhism, Jaina, Charvaka, Aajivika
4. Naastika Theist philosophies: Sikhism, Sufism.