The Third Buddhist council was convened in about 250 BCE at Asokarama in Patiliputta, under the patronage of Emperor Ashoka. The reason for convening the Third Buddhist Council is reported to have been to rid the Sangha of corruption and bogus monks who held heretical views. It was presided over by the Elder Moggaliputta Tissa and one thousand monks participated in the Council.
The Theravadin account of the background to the Third Council is as follows: Emperor Ashoka was crowned in the two hundred and eighteenth year after the Buddha's Mahaparinibbāna. At first he paid only token homage to the Dhamma and the Sangha and also supported members of other religious sects as his father had done before him. However, all this changed when he met the pious novice-monk Nigrodha who preached him the Appamada-vagga. Thereafter he ceased supporting other religious groups and his interest in and devotion to the Dhamma deepened. He used his enormous wealth to build, it is said, eighty-four thousand pagodas and viharas and to lavishly support the bhikkhus with the four requisites. His son Mahinda and his daughter Sanghamitta were ordained and admitted to the Sangha.
Eventually, his generosity was to cause serious problems within the Sangha. In time the order was infiltrated by many unworthy men, holding heretical views and who were attracted to the order because of the Emperor's generous support and costly offerings of food, clothing, shelter and medicine. Large numbers of faithless, greedy men espousing wrong views tried to join the order but were deemed unfit for ordination.
Despite this they seized the chance to exploit the Emperor's generosity for their own ends and donned robes and joined the order without having been ordained properly. Consequently, respect for the Sangha diminished. When this came to light some of the genuine monks refused to hold the prescribed purification or Uposatha ceremony in the company of the corrupt, heretical monks.
When the Emperor heard about this he sought to rectify the situation and dispatched one of his ministers to the monks with the command that they perform the ceremony. However, the Emperor had given the minister no specific orders as to what means were to be used to carry out his command. The monks refused to obey and hold the ceremony in the company of their false and 'thieving' companions (Pali, theyya-sinivāsaka).
In desperation the angry minister advanced down the line of seated monks and drawing his sword, beheaded all of them one after the other until he came to the King's brother, Tissa who had been ordained. The horrified minister stopped the slaughter and fled the hall and reported back to the Emperor. Asoka was deeply grieved and upset by what had happened and blamed himself for the killings. He sought Thera Moggaliputta Tissa's counsel. He proposed that the heretical monks be expelled from the order and a third Council be convened immediately.
So it was that in the seventeenth year of the Emperor's reign the Third Council was called. Thera Moggaliputta Tissa headed the proceedings and chose one thousand monks from the sixty thousand participants for the traditional recitation of the Dhamma and the Vinaya, which went on for nine months. The Emperor, himself questioned monks from a number of monasteries about the teachings of the Buddha. Those who held wrong views were exposed and expelled from the Sangha immediately. In this way the Bhikkhu Sangha was purged of heretics and bogus bhikkhus.
According to the Pali account, the Elder Moggaliputta Tissa, in order to refute a number of heresies and ensure the Dhamma was kept pure, complied a book during the council called the Kathavatthu. This book consists of twenty-three chapters, and is a collection of discussions on the points of controversy. It gives refutations of the 'heretical' views held by various Buddhist sects on matters philosophical. The Kathavatthu is the fifth of the seven books of the Abhidhamma Pitaka. However, the historicity of this has been questioned, as the account preserved in the San Jian Lu Pi Po Sho (Sudassanavinayavibhasha), although otherwise almost identical, does not mention the Kathavatthu.
Moggaliputtatissa told Ashoka that the doctrine taught by the Buddha was the Vibhajjavada, the Doctrine of Analysis. This term is used in various senses, and it is not clear exactly what it meant in this context. Traditionally, however, the Sri Lankan Theravadins and other mainland schools of Early Buddhism identified themselves as Vibhajjavada.