In the earliest period of Buddhism more attention was paid to the four noble truths than to systematic metaphysics. What was sorrow, what was the cause of sorrow, what was the cessation of sorrow and what could lead to it? The doctrine of _pa@ticcasamuppada_ was offered only to explain how sorrow came in and not with a view to the solving of a metaphysical problem. The discussion of ultimate metaphysical problems, such as whether the world was eternal or non-eternal, or whether a Tathagata existed after death or not, were considered as heresies in early Buddhism. Great emphasis was laid on sila, samadhi and panna and the doctrine that there was no soul. The Abhidhammas hardly give us any new philosophy which was not contained in the Suttas. They only elaborated the materials of the suttas with enumerations and definitions. With the evolution of Mahayana scriptures from some time about 200 B.C. the doctrine of the non-essentialness and voidness of all _dhammas_ began to be preached. This doctrine, which was taken up and elaborated by Nagarjuna, Aryyadeva, Kumarajiva and Candrakirtti, is more or less a corollary from the older doctrine of Buddhism. If one could not say whether the world was eternal or non-eternal, or whether a Tathagata existed or did not exist after death, and if there was no permanent soul and all the dhammas were changing, the only legitimate way of thinking about all things appeared to be to think of them as mere void and non-essential appearances. These appearances appear as being mutually related but apart from their appearance they have no other essence, no being or reality. The Tathata doctrine which was preached by As'vagho@sa oscillated between the position of this absolute non-essentialness of all dhammas and the Brahminic idea that something existed as the background of all these non-essential dhammas. This he called tathata, but he could not consistently say that any such permanent entity could exist. The Vijnanavada doctrine which also took its rise at this time appears to me to be a mixture of the S'unyavada doctrine and the Tathata doctrine; but when carefully examined it seems to be nothing but S'unyavada, with an attempt at explaining all the observed phenomena. If everything was non-essential how did it originate? Vijnanavada proposes to give an answer, and says that these phenomena are all but ideas of the mind generated by the beginningless vasana (desire) of the mind. The difficulty which is felt with regard to the Tathata doctrine that there must be some reality which is generating all these ideas appearing as phenomena, is the same as that in the Vijnanavada doctrine. The Vijnanavadins could not admit the existence of such a reality, but yet their doctrines led them to it. They could not properly solve the difficulty, and admitted that their doctrine was some sort of a compromise with the Brahminical doctrines of heresy, but they said that this was a compromise to make the doctrine intelligible to the heretics; in truth however the reality assumed in the doctrine was also non-essential. The Vijnanavada literature that is available to us is very scanty and from that we are not in a position to judge what answers Vijnanavada could give on the point. These three doctrines developed almost about the same time and the difficulty of conceiving s'unya (void), tathata,
(thatness) and the alayavijnana of Vijnanavada is more or less the same.
The Tathata doctrine of As'vagho@sa practically ceased with him. But the S'unyavada and the Vijnanavada doctrines which originated probably about 200 B.C. continued to develop probably till the eighth century A.D. Vigorous disputes with S'unyavada doctrines are rarely made in any independent work of Hindu philosophy, after Kumarila and S'a@nkara. From the third or the fourth century A.D. some Buddhists took to the study of systematic logic and began to criticize the doctrine of the Hindu logicians. Di@nnaga the Buddhist logician (500 A.D.) probably started these hostile criticisms by trying to refute the doctrines of the great Hindu logician Vatsyayana, in his Prama@nasamuccaya. In association with this logical activity we find the activity of two other schools of Buddhism, viz. the Sarvastivadins (known also as Vaibha@sikas) and the Sautrantikas. Both the Vaibha@sikas and the Sautrantikas accepted the existence of the external world, and they were generally in conflict with the Hindu schools of thought Nyaya-Vais'e@sika and Sa@mkhya which also admitted the existence of the external world. Vasubandhu (420-500 A.D.) was one of the most illustrious names of this school. We have from this time forth a number of great Buddhist thinkers such as Yas'omitra (commentator of Vasubandhu's work), Dharmmakirtti (writer of Nyayabindu 635 A.D.), Vinitadeva and S'antabhadra (commentators of Nyayabindu), Dharmmottara (commentator of Nyayabindu 847 A.D.), Ratnakirtti (950 A.D.), Pa@n@dita As'oka, and Ratnakara S'anti, some of whose contributions have been published in the _Six Buddhist Nyaya Tracts_, published in Calcutta in the _Bibliotheca Indica_ series. These Buddhist writers were mainly interested in discussions regarding the nature of perception, inference, the doctrine of momentariness, and the doctrine of causal efficiency (_arthakriyakaritva_) as demonstrating the nature of existence. On the negative side they were interested in denying the ontological theories of Nyaya and Sa@mkhya with regard to the nature of class-concepts, negation, relation of whole and part, connotation of terms, etc. These problems hardly attracted any notice in the non-Sautrantika and non-Vaibha@sika schools of Buddhism of earlier times. They of course agreed with the earlier Buddhists in denying the existence of a permanent soul, but this they did with the help of their doctrine of causal efficiency. The points of disagreement between Hindu thought up to S'a@nkara (800 A.D.) and Buddhist thought till the time of S'a@nkara consisted mainly in the denial by the Buddhists of a permanent soul and the permanent external world. For Hindu thought was more or less realistic, and even the Vedanta of S'a@nkara admitted the existence of the permanent external world in some sense. With S'a@nkara the forms of the external world were no doubt illusory, but they all had a permanent background in the Brahman, which was the only reality behind all mental and the physical phenomena. The Sautrantikas admitted the existence of the external world and so their quarrel with Nyaya and Sa@mkhya was with regard to their doctrine of momentariness; their denial of soul and their views on the different ontological problems were in accordance with their doctrine of momentariness. After the twelfth century we do not hear much of any new disputes with the Buddhists. From this time the disputes were mainly between the different systems of Hindu philosophers, viz. Nyaya, the Vedanta of the school of S'a@nkara and the Theistic Vedanta of Ramanuja, Madhva, etc.