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),

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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.