It is difficult to say how the systems were originally formulated, and what were the influences that led to it. We know that a spirit of philosophic enquiry had already begun in the days of the earliest Upani@sads. The spirit of that enquiry was that the final essence or truth was the atman, that a search after it was our highest duty, and that until we are ultimately merged in it we can only feel this truth and remain uncontented with everything else and say that it is not the truth we want, it is not the truth we want (_neti neti_). Philosophical enquires were however continuing in circles other than those of the Upani@sads. Thus the Buddha who closely followed the early Upani@sad period, spoke of and enumerated sixty-two kinds of heresies [Footnote ref 92], and these can hardly be traced in the Upani@sads. The Jaina activities were also probably going on contemporaneously but in the Upani@sads no reference to these can be found. We may thus reasonably suppose that there were different forms of philosophic enquiry in spheres other than those of the Upani@sad sages, of which we have but scanty records. It seems probable that the Hindu systems of thought originated among the sages who though attached chiefly to the Upani@sad circles used to take note of the discussions and views of the antagonistic and heretical philosophic circles. In the assemblies of these sages and their pupils, the views of the heretical circles were probably discussed and refuted. So it continued probably for some time when some illustrious member of the assembly such as Gautama or Kanada collected the purport of these discussions on various topics and problems, filled up many of the missing links, classified and arranged these in the form of a system of philosophy and recorded it in sutras. These sutras were intended probably for people who had attended the elaborate oral discussions and thus could easily follow the meaning of the suggestive phrases contained in the aphorisms. The sutras thus contain sometimes allusions to the views of the rival schools and indicate the way in which they could be refuted. The commentators were possessed of the general drift of the different discussions alluded to and conveyed from generation to generation through an unbroken chain of succession of teachers and pupils. They were however free to supplement these traditionary explanations with their ownviews or to modify and even suppress such of the traditionary views with which they did not agree or which they found it difficult to maintain. Brilliant oppositions from the opposing schools often made it necessary for them to offer solutions to new problems unthought of before, but put forward by some illustrious adherent of a rival school. In order to reconcile these new solutions with the other parts of the system, the commentators never hesitated to offer such slight modifications of the doctrines as could harmonize them into a complete whole. These elaborations or modifications generally developed the traditionary system, but did not effect any serious change in the system as expounded by the older teachers, for the new exponents always bound themselves to the explanations of the older teachers and never contradicted them. They would only interpret them to suit their own ideas, or say new things only in those cases where the older teachers had remained silent. It is not therefore possible to describe the growth of any system by treating the contributions of the individual commentators separately. This would only mean unnecessary repetition. Except when there is a specially new development, the system is to be interpreted on the basis of the joint work of the commentators treating their contributions as forming one whole.
The fact that each system had to contend with other rival systems in order to hold its own has left its permanent mark upon all the philosophic literatures of India which are always written in the form of disputes, where the writer is supposed to be always faced with objections from rival schools to whatever he has got to say. At each step he supposes certain objections put forth against him which he answers, and points out the defects of the objector or shows that the objection itself is ill founded. It is thus through interminable byways of objections, counter-objections and their answers that the writer can wend his way to his destination. Most often the objections of the rival schools are referred to in so brief a manner that those only who know the views can catch them. To add to these difficulties the Sanskrit style of most of the commentaries is so condensed and different from literary Sanskrit, and aims so much at precision and brevity, leading to the use of technical words current in the diverse systems, that a study of these becomes often impossible without the aid of an expert preceptor; it is difficult therefore for all who are not widely read in all the different systems to follow any advanced work of any particular system, as the deliberations of that particular system are expressed in such close interconnection with the views of other systems that these can hardly be understood without them. Each system of India has grown (at least in particular epochs) in relation to and in opposition to the growth of other systems of thought, and to be a thorough student of Indian philosophy one should study all the systems in their mutual opposition and relation from the earliest times to a period at which they ceased to grow and came to a stop--a purpose for which a work like the present one may only be regarded as forming a preliminary introduction.
Besides the sutras and their commentaries there are also independent treatises on the systems in verse called _karikas_, which try to summarize the important topics of any system in a succinct manner; the _Sa@mkhya karika_ may be mentioned as a work of this kind. In addition to these there were also long dissertations, commentaries, or general observations on any system written in verses called the varttikas; the _S'lokavarttika_, of Kumarila or the _Varttika_ of Sures'vara may be mentioned as examples. All these of course had their commentaries to explain them. In addition to these there were also advanced treatises on the systems in prose in which the writers either nominally followed some selected sutras or proceeded independently of them. Of the former class the _Nyayamanjari_ of Jayanta may be mentioned as an example and of the latter the _Pras'astapada bha@sya_, the _Advaitasiddhi_ of Madhusudana Sarasvati or the _Vedanta-paribha@sa_ of Dharmarajadhvarindra. The more remarkable of these treatises were of a masterly nature in which the writers represented the systems they adhered to in a highly forcible and logical manner by dint of their own great mental powers and genius. These also had their commentaries to explain and elaborate them. The period of the growth of the philosophic literatures of India begins from about 500 B.C. (about the time of the Buddha) and practically ends in the later half of the seventeenth century, though even now some minor publications are seen to come out.