It is probable that the Nyaya philosophy arose in an atmosphere of continued disputes and debates; as a consequence of this we find here many terms related to debates which we do not notice in any other system of Indian philosophy. These are _tarka_, _nir@naya_, _vada_, _jalpa_, _vita@n@da_, _hetvabhasa_, _chala_, _jati_ and _nigrahasthana_.
Tarka means deliberation on an unknown thing to discern its real nature; it thus consists of seeking reasons in favour of some supposition to the exclusion of other suppositions; it is not inference, but merely an oscillation of the mind to come to a right conclusion. When there is doubt (_sa@ms'aya_) about the specific nature of anything we have to take to tarka. Nir@naya means the conclusion to which we arrive as a result of tarka. When two opposite parties dispute over their respective theses, such as the doctrines that there is or is not an atman, in which each of them tries to prove his own thesis with reasons, each of the theses is called a _vada_. Jalpa means a dispute in which the disputants give wrangling rejoinders in order to defeat their respective opponents. A jalpa is called a _vita@n@da_ when it is only a destructive criticism which seeks to refute the opponent's doctrine without seeking to establish or formulate any new doctrine. Hetvabhasas are those which appear as hetus but are really not so. _Nyaya_ sutras enumerate five fallacies (_hetvabhasas_) of the middle (hetu): _savyabhicara_ (erratic), _viruddha_ (contradictory), _prakara@nasama_ (tautology), _saddhyasama_ (unproved reason) and _kalatita _(inopportune). Savyabhicara is that where the same reason may prove opposite conclusions (e.g. sound is eternal because it is intangible like the atoms which are eternal, and sound is non-eternal because it is intangible like cognitions which are non-eternal); viruddha is that where the reason opposes the premiss to be proved (e.g. a jug is eternal, because it is produced); prakara@nasama is thatwhere the reason repeats the thesis to be proved in another form (e.g. sound is non-eternal because it has not the quality of eternality); sadhyasama is that where the reason itself requires to be proved (e.g. shadow is a substance because it has motion, but it remains to be proved whether shadows have motion or not); kalatita is a false analogy where the reason fails because it does not tally with the example in point of time. Thus one may argue that sound is eternal because it is the result of contact (stick and the drum) like colour which is also a result of contact of light and the object and is eternal. Here the fallacy lies in this, that colour is simultaneous with the contact of light which shows what was already there and only manifested by the light, whereas in the case of sound it is produced immediately after the contact of the stick and drum and is hence a product and hence non-eternal. The later Nyaya works divide savyabhicara into three classes,
(1) sadhara@na or common (e.g. the mountain is fiery because it is an object of knowledge, but even a lake which is opposed to fire is also an object of knowledge),
(2) asadhara@na or too restricted (e.g. sound is eternal because it has the nature of sound; this cannot be a reason for the nature of sound exists only in the sound and nowhere else), and
(3) anupasa@mharin or unsubsuming (e.g. everything is non-eternal, because they are all objects of knowledge; here the fallacy lies in this, that no instance can be found which is not an object of knowledge and an opposite conclusion may also be drawn). The fallacy _satpratipak@sa_ is that in which there is a contrary reason which may prove the opposite conclusion (e.g. sound is eternal because it is audible, sound is non-eternal because it is an effect). The fallacy _asiddha_ (unreal) is of three kinds
(1) _as'rayasiddha_ (the lotus of the sky is fragrant because it is like other lotuses; now there cannot be any lotus in the sky),
(2) _svarupasiddha_ (sound is a quality because it is visible; but sound has no visibility),
(3) _vyapyatvasiddha_ is that where the concomitance between the middle and the consequence is not invariable and inevitable; there is smoke in the hill because there is fire; but there may be fire without the smoke as in a red hot iron ball, it is only green-wood fire that is invariably associated with smoke. The fallacy _badhita_ is that which pretends to prove a thesis which is against direct experience, e.g. fire is not hot because it is a substance. We have already enumerated the fallacies counted by Vais'e@sika. Contrary to Nyaya practice Pras'astapada counts the fallacies of the example. Di@nnaga also counted fallacies of example (e.g. sound is eternal, because it is incorporeal, that which is incorporeal is eternal as the atoms; but atoms are not incorporeal) and Dharmakirtti counted also the fallacies of the pak@sa (minor); but Nyaya rightly considers that the fallacies of the middle if avoided will completely safeguard inference and that these are mere repetitions. Chala means the intentional misinterpretation of the opponent's arguments for the purpose of defeating him. Jati consists in the drawing of contradictory conclusions, the raising of false issues or the like with the deliberate intention of defeating an opponent. Nigrahasthana means the exposure of the opponent's argument as involving self-contradiction, inconsistency or the like, by which his defeat is conclusively proved before the people to the glory of the victorious opponent. As to the utility of the description of so many debating tricks by which an opponent might be defeated in a metaphysical work, the aim of which ought to be to direct the ways that lead to emancipation, it is said by Jayanta in his _Nyayamanjari_ that these had to be resorted to as a protective measure against arrogant disputants who often tried to humiliate a teacher before his pupils. If the teacher could not silence the opponent, the faith of the pupils in him would be shaken and great disorder would follow, and it was therefore deemed necessary that he who was plodding onward for the attainment of mok@sa should acquire these devices for the protection of his own faith and that of his pupils. A knowledge of these has therefore been enjoined in the Nyaya sutra as being necessary for the attainment of salvation [Footnote ref 527].