Even as the naastikas are those who repudiate the Vedas, the aastikas are those who accept the authority of the Veda, the varna and the aashrama system. Due to this and also because a good majority of the aastika authors were Brahmins and used Sanskrit as the lingua franca, the aastika systems are also sometimes referred to as the Brahmanical systems.

We do not fully agree with the popular opinion that the aastika or the orthodox schools sprang into existence only after the attacks on the orthodoxy by the heterodox (naastika) schools. The Indian mind is extremely inquisitive and revels in disputing, debating and analyzing. This can be observed in the Upanishads themselves, where large groups of scholars assemble in Parishads and debate over philosophical issues. Not only the Upanishads, but the later sutra and its commentarial literature with its long history of polemical matches bears ample witness to this inclination towards debating. The Vedas and the Upanishads do not teach any single doctrine about reality. The teachings are varied and hence interpretations are also bound to be varied. Where there're varied interpretations there are also bound to be disputes between the interpreters and this, in a debating environment, forces them to systematize their doctrines and set forth logical defenses for them. So though we do not fully dispute the point that the naastika schools might have been one of the causes which gave rise to the aastika schools, we would also like to point out that the proto-orthodox streams themselves had the necessary internal dynamism to give rise to the later aastika streams. And quite in line with this the aastika schools engage in polemics not only against naastikas, but amongst themselves as well. Even as per Indological opinion if not all aastika streams, Saamkhya and Vaisheshika were definitely older than all naastika streams. And according to Max Mueller the Brahma Sutras are pre-Buddhist. And as we all know "Yoga is older than Patanjali" - so it is also quite safe to assume that though the aastika streams existed in their primitive forms, with increasing criticism not only from the naastikas but other aastika schools as well, each school enhanced and perfected its philosophy and these new works over a period of time replaced the older ones.

Darshana

Faced with increasing criticism the orthodoxy found it hard to fall back on some supernatural revelation for defense of their metaphysical schemes and recognized the need to reconcile their metaphysical ideal with experience and reason. All schools of philosophy both aastika as well as naastika were termed as Darshanas. The term signifies the willingness of schools to reconcile their metaphysical and spiritual ideal with reason. Quite in contrast to the mystical and religious nature of early Vedic literature (the Vedas, the Upanishads and the Gita), cold criticism and analysis dominate the literature of the Darshanas. The effort is on mainly to codify the doctrines and set forth logical defenses for them. The critical side of philosophy becomes as important as metaphysical speculation.

Amongst the aastika darshanas six became more famous than others. They are Kapila's Saamkhya, Patanjali's Yoga, Kanaada's Vaisheshika, Gautama's Nyaaya, Jaimini's Purva Mimaamsaa and Baadaraayanaa's Uttara Mimaamsaa or the Vedaanta. Though each school (may have) started out individually, over a period of time they merged in pairs i.e, Saamkhya and Yoga, Nyaaya and Vaisheshika, Purva and Uttara Mimaamsaa. The pairs are considered as allied systems. The coming together was facilitated by each member of a pair sharing with the other member, a lot common ideas both in metaphysics and psychology, ontology and epistemology. While Saamkhya's emphasis is more on the metaphysics, the Yoga works on psychology. Likewise while Vaisheshika concentrates on metaphysics, Nyaaya works on epistemology and logic. Purva and Uttara Mimaamsaa are natural allies as they are interpreters of the early and the latter portions of the Vedas. Authors of each school frequently quote from the works of the sister school.

Sutras

The principal tenets of the darshanas are stated in the form of Sutras or short aphorisms. Since they lacked the technology to print books and also to keep their teachings secret, the followers of the darshanas relied on the ancient Vedic practice of committing the texts to memory. To facilitate this the sutras had to be as short as possible, free from doubt, able to bring out the essential meaning and put to end many doubts. They must be to the point and free from error. They try to avoid all unnecessary repetition and employ great economy of words. This extreme conciseness makes them cryptic and difficult to understand without a commentary.

But the Sutras are not to be taken as the point of origin of any darshana. The Sutras presuppose a period of gestation and formation. They are not the views of a single thinker. Generations of thinkers were developing these views before they were systematized and put down in the form of Sutras. So the authors of the Sutras are not the founders of the system in the strictest sense, but only their compilers or formulators. This among other things also makes it very difficult to assign a date of origin for any darshana. And since it was the oral tradition which was the repository of these philosophical views, it is almost certain that a great number of important works perished and that which has come down to us may not even be pure.

The darshanas grew side by side, disputing and debating with each other and enhancing their doctrines. Each darshana has along with its Sutras, commentaries, glosses, expositions and explanatory compendia. Commentaries upon commentaries were written by the followers of each system perfecting their schools philosophy and also engaging in polemics with rival schools. The commentaries generally use the form of the dialogue where the commentator shows the relation of the view he's expounding in contrast to the views of rival schools. The ideas are re-stated and their superiority to other conceptions established.

Though the commentators freely enhance, modify and perfect the doctrines, still they claim the result as the teaching of the original systamatizer. The greatest of the Indian thinkers, whatever be their original contribution, still humbly state that they're only preserving and teaching the philosophy of the tradition (sampradhaayam). Each system has always grown in relation to the other systems and the development of the systems has been in progress till the present day, the successive interpreters defending the tradition against the attacks of its opponents.

Common ideas

The aastikas agree with the Jainaas and the Bauddhas on certain fundamentals :

1. Phenomenal life (samsaara) is ultimately suffering and there's an endless cycle of rebirths.
2. We reap the fruits of our past acts - karma - and this is what keeps samsaara going in an endless cycle.
3. Liberation (moksha or nirvaana) is escape from the cycle of rebirths, which is to be obtained even while we live.
4. So all these systems, except the Purva Mimaasaa claim to be moksha shaastras or the texts which teach the way to liberation.
5. Before the rise of Vedaanta, all the ethical schools, aastika as well as naastika, believed that knowledge - either of the true nature of our own selves and the world, or of the non-substantiality of our own selves and the world - would lead to liberation. In short we can liberate ourselves irrespective of whether God exists/helps or not. Advaita Vedaanta though it stresses that it is only by knowledge of one's own self that liberation can be obtained, it also states that liberation is ultimately through God's grace. The apparent contradiction can be resolved by understanding that according to Advaita the true self of man is none other than Reality itself, whose nature is pure consciousness/knowledge. The schools of Bhakti Vedaanta on the other hand, though they do agree that it is knowledge that liberates, still do not place so much emphasis on self-effort. They teach that ultimately devotion (bhakti) and surrender to the Lord is what will lead to His grace, which in turn will result in the dawning of true knowledge of one's own self and thus effect liberation.
6. All schools except the Lokaayatas enjoin the practice of control (of the body, mind and senses), charity and compassion.

Now let's look at the common ideas among the aastikas :

1. All the aastika schools accept the authority of the Vedas and hold it as a valid means of knowledge.
2. All of them with varying degrees of acceptance, accept the chatur varna and the aashrama systems.
3. All the aastikas believe in the objective reality of the world. The world exists apart from our mind. The material world is either due to the evolution of praakriti or the combination of atoms or due to maya.
4. The aastikas also hold that samsaara is an endless cycle. It has neither beginning nor end. Vast periods of creation, maintenance and dissolution follow each other in endless succession. Dissolution doesn't mean annihilation nor does creation mean fresh creation. Dissolution either means the return of matter to its primal state or the going apart of atoms. Creation means the evolution of matter or the coming together of atoms due to past karma.
5. Except for Kumaarilla Bhatta and his followers, all aastika thinkers hold that there's an eternal, unchanging self (Atman or Purusha) in man. It is his true self/identity. Samsaara is transience/change and hence suffering. So liberation from samsaara lies in the changeless self, which is unaffected by all the changes happening around it in the world. Man is caught up in samsaara as he is ignorant of his own true nature. Our true self is to be known either by intuition, either with or without God's grace. Kumaarilla's view of the self is quite similar to that of the Jainaas who hold that the self is affected by karma and so has to evolve/purify to attain liberation.