When we compare the _Nyaya sutras_ with the _Vais'e@sika sutras_ we find that in the former two or three differentstreams of purposes have met, whereas the latter is much more homogeneous. The large amount of materials relating to debates treated as a practical art for defeating an opponent would lead one to suppose that it was probably originally compiled from some other existing treatises which were used by Hindus and Buddhists alike for rendering themselves fit to hold their own in debates with their opponents [Footnote ref 447]. This assumption is justified whenwe compare the futilities (jati) quibbles (chala), etc., relating to disputations as found in the _Nyaya sutra_ with those that are found in the medical work of Caraka (78 A.D.), III. viii. There are no other works in early Sanskrit literature, excepting the _Nyaya sutra_ and _Caraka-sa@mhita_ which have treated of these matters. Caraka's description of some of the categories (e.g. d@r@s@tanta, prayojana, pratijna and vita@n@da) follows very closely the definitions given of those in the _Nyaya sutras_. There are others such as the definitions of jalpa, chala, nigrahasthana, etc., where the definitions of two authorities differ more. There are some other logical categories mentioned in Caraka (e.g. _prati@s@thapana, jijnasa, vyavasaya, vakyado@sa, vakyapras'a@msa, upalambha, parihara, abhyanujna_, etc.) which are not found in the _Nyaya sutra_ [Footnote ref 448]. Again, the various types of futilities (jati) and points of opponent's refutation (nigrahasthana) mentioned in the _Nyaya sutra_ are not found in _Caraka_. There are some terms which are found in slightly variant forms in the two works, e.g. _aupamya_ in _Caraka, upamana_ in _Nyaya sutra, arthapatti_ in _Nyaya sutra_ and _arthaprapti_ in _Caraka_. Caraka does not seem to know anything about the Nyaya work on this subject, and it is plain that the treatment of these terms of disputations in the _Caraka_ is much simpler and less technical than what we find in the _Nyaya sutras_. If we leave out the varieties of jati and nigrahasthana of the fifth book, there is on the whole a great agreement between the treatment of Caraka and that of the _Nyaya sutras_. It seems therefore in a high degree probable that both Caraka and the _Nyaya sutras_ were indebted for their treatment of these terms of disputation to some other earlier work. Of these, Caraka's compilation was earlier, whereas the compilation of the _Nyaya sutras_ represents a later work when a hotter atmosphere of disputations had necessitated the use of more technical terms which are embodied in this work, but which were not contained in the earlier work. It does not seem therefore that this part of the work could have been earlier than the second century A.D. Another stream flowing through the _Nyaya sutras_ is that of a polemic against the doctrines which could be attributed to the Sautrantika Buddhists, the Vijnanavada Buddhists, the nihilists, the Sa@mkhya, the Carvaka, and some other unknown schools of thought to which we find nofurther allusion elsewhere. The _Vais'e@sika sutras_ as we have already seen had argued only against the Mima@msa, and ultimately agreed with them on most points. The dispute with Mima@msa in the _Nyaya sutras_ is the same as in the Vais'e@sika over the question of the doctrine of the eternality of sound. The question of the self-validity of knowledge (_svata@h prama@nyavada_)and the akhyati doctrine of illusion of the Mima@msists, which form the two chief points of discussion between later Mima@msa and later Nyaya, are never alluded to in the _Nyaya sutras_. The advocacy of Yoga methods (_Nyaya sutras_, IV.ii.38-42 and 46) seems also to be an alien element; these are not found in Vais'e@sika and are not in keeping with the general tendency of the _Nyaya sutras_, and the Japanese tradition that Mirok added them later on as Mahamahopadhyaya Haraprasada S'astri has pointed out [Footnote ref 449] is not improbable.
The _Vais'e@sika sutras_, III.i.18 and III.ii.1, describe perceptional knowledge as produced by the close proximity of the self (atman), the senses and the objects of sense, and they also adhere to the doctrine, that colour can only be perceived under special conditions of sa@mskara (conglomeration etc.). The reason for inferring the existence of manas from the non-simultaneity (_ayaugapadya_) of knowledge and efforts is almost the same with Vais'e@sika as with Nyaya. The _Nyaya sutras_ give a more technical definition of perception, but do not bring in the questions of sa@mskara or udbhutarupavattva which Vais'e@sika does. On the question of inference Nyaya gives three classifications as purvavat, s'e@savat and samanyatod@r@s@ta, but no definition. The _Vais'e@sika sutras_ do not know of these classifications, and give only particular types or instances of inference (V.S. III. i. 7-17, IX. ii. 1-2, 4-5). Inference is said to be made when a thing is in contact with another, or when it is in a relation of inherence in it, or when it inheres in a third thing; one kind of effect may lead to the inference of another kind of effect, and so on. These are but mere collections of specific instances of inference without reaching a general theory. The doctrine of vyapti (concomitance of _hetu_ (reason) and _sadhya_ (probandum)) which became so important in later Nyaya has never been properly formulated either in the _Nyaya sutras_ or in the Vais'e@sika. _Vais'e@sika sutra_, III. i. 24, no doubt assumes the knowledge of concomitance between hetu and sadhya (_prasiddhipurvakatvat apades'asya_),but the technical vyapti is not known, and the connotation of the term _prasiddhipurvakatva_ of Vais'e@sika seems to be more loose than the term _vyapti_ as we know it in the later Nyaya. The _Vais'e@sika sutras_ do not count scriptures (_s'abda_) as a separate prama@na, but they tacitly admit the great validity of the Vedas. With _Nyaya sutras_ s'abda as a prama@na applies not only to the Vedas, but to the testimony of any trustworthy person, and Vatsyayana says that trustworthy persons may be of three kinds _@r@si, arya_ and _mleccha_ (foreigners). Upamana which is regarded as a means of right cognition in Nyaya is not even referred to in the _Vais'e@sika sutras_. The _Nyaya sutras_ know of other prama@nas, such as _arthapatti, sambhava_ and _aitihya_, but include them within the prama@nas admitted by them, but the _Vais'e@sika sutras_ do not seem to know them at all [Footnote ref 450]. The _Vais'e@sika sutras_ believe in the perception of negation (abhava) through the perception of the locus to which such negation refers (IX. i. 1-10). The _Nyaya sutras_ (II. ii. 1, 2, 7-12) consider that abhava as non-existence or negation can be perceived; when one asks another to "bring the clothes which are not marked," he finds that marks are absent in some clothes and brings them; so it is argued that absence or non-existence can be directly perceived [Footnote ref 451]. Though there is thus an agreement between the Nyaya and the _Vais'e@sika sutras_ about the acceptance of abhava as being due to perception, yet their method of handling the matter is different. The _Nyaya sutras_ say nothing about the categories of _dravya, gu@na, karma, vis'e@sa_ and _samavaya_ which form the main subjects of Vais'e@ska discussions [Footnote ref 452]. The _Nyaya sutras_ take much pains to prove the materiality of the senses. But this question does not seem to have been important with Vais'e@sika. The slight reference to this question in VIII. ii. 5-6 can hardly be regarded as sufficient. The _Vais'e@sika sutras_ do not mention the name of "Is'vara," whereas the _Nyaya sutras_ try to prove his existence on eschatological grounds. The reasons given in support of the existence of self in the _Nyaya sutras_ are mainly on the ground of the unity of sense-cognitions and the phenomenon of recognition, whereas theVaisesika lays its main emphasis on self-consciousness as a fact of knowledge. Both the Nyaya and the _Vais'e@sika sutras_ admit the existence of atoms, but all the details of the doctrine of atomic structure in later Nyaya-Vais'e@sika are absent there. The Vai'se@sika calls salvation _ni@hs'reyasa_ or _mok@sa_ and the Nyaya _apavarga_. Mok@sa with Vais'e@sika is the permanent cessation of connection with body; the apavarga with Nyaya is cessation of pain [Footnote ref 453]. In later times the main points of difference between the Vais'e@sika and Nyaya are said to lie with regard to theory of the notion of number, changes of colour in the molecules by heat, etc. Thus the former admitted a special procedure of the mind by which cognitions of number arose in the mind (e.g. at the first moment there is the sense contact with an object, then the notion of oneness, then from a sense of relativeness--apek@sabuddhi--notion of two, then a notion of two-ness, and then the notion of two things); again, the doctrine of pilupaka (changes of qualities by heat are produced in atoms and not in molecules as Nyaya held) was held by Vais'e@sika, which the Naiyayikas did not admit [Footnote ref 454]. But as the _Nyaya sutras_ are silent on these points, it is not possible to say that such were really the differences between early Nyaya and early Vaise@sika. These differences may be said to hold between the later interpreters of Vais'e@sika and the later interpreters of Nyaya. The Vais'e@sika as we find it in the commentary of Pras'astapada (probably sixth century A.D.), and the Nyaya from the time of Udyotakara have come to be treated as almost the same system with slight variations only. I have therefore preferred to treat them together. The main presentation of the Nyaya-Vais'e@sika philosophy in this chapter is that which is found from the sixth century onwards.