Early in the fifth century, the explanations that told a monk in Sri Lanka what the Buddha's discourses meant were written in Sinhala, the island's own language. The discourses themselves were in Pāli, the scriptural language of the Theravāda, the school of the elders. The commentaries had been built up over centuries at the Mahāvihāra, the great monastery of Anurādhapura, and a monk who knew no Sinhala could not use them. Buddhaghosa, an Indian monk who came to that monastery, put them into Pāli. In the opening verses of his commentary on the middle-length discourses he says what he did: he took away the Sinhala, set the substance in the language of the scriptures, dropped what was repeated, and kept to the teaching of the Mahāvihāra's elders. None of the Sinhala originals survives; scholars know them only from quotations in the Pāli commentaries.
The monastery had reason to want its reading fixed. The Abhayagiri, a second great monastery in the same city, had broken away in the first century BCE and is thought to have been more open to Mahāyāna teachings than the Mahāvihāra. Faxian, a Chinese pilgrim who stayed on the island early in the fifth century, counted five thousand monks at the Abhayagiri and three thousand at the Mahāvihāra. An older manual of the path also circulated, the Vimuttimagga of the teacher Upatissa, and the Indian scholar P. V. Bapat argued that Buddhaghosa wrote some paragraphs of his own with that book and the Abhayagiri in mind.
Of his life, his own books record little. Their closing verses name the elders who asked for each, and two add a memory of India: the elder who asked for the commentary on the numbered discourses had lived with him at Kāñcī, today's Kanchipuram in south India; the one who asked for the middle-length commentary, at a place called Mayūradūtapaṭṭana. The fuller story belongs to the tradition. The Cūḷavaṃsa, a Sri Lankan chronicle continued around the thirteenth century, tells that he was born a Brahmin near Bodh Gaya, where the Buddha attained awakening, mastered the Vedas, lost a debate to the monk Revata and joined the order. Revata sent him to the island. There, it says, the elders tested him with two verses before they would open their commentaries, and he answered with a whole book; deities hid it twice, he wrote it out again each time, and the three copies agreed. Historians date his arrival to the reign of King Mahānāma of Anurādhapura, in the first decades of the century. Scholars such as Oskar von Hinüber, a German Indologist, think he came from the south of India, and that the chronicle moved his birthplace closer to the places of the Buddha's life. His name means "voice of the Buddha".
The book in that story is The Path of Purification (Visuddhimagga). It takes one verse in which the Buddha says how a person untangles the tangle within and without, and builds the whole path from it in three parts: sīla, moral conduct; samādhi, concentration, trained on forty subjects of meditation; and paññā, understanding. The path climbs through seven stages of purification; the last five fall in the chapters on understanding. Its claim is that what a person takes for a self is a run of bodily and mental events, each arising from conditions and passing, and that seeing this clearly, stage by stage, is what sets the mind free.
His commentaries tell the occasion of each discourse, gloss its words one by one, and send the reader back to this book whenever meditation comes up. Maria Heim, a scholar of religion at Amherst College, argues that he treated the Buddha's words as beyond measure, so that explanation could always go further. The tradition credits him with commentaries on the Abhidhamma, the scriptures' analysis of mind and matter, and on parts of the Minor Collection; modern scholars accept the Visuddhimagga and the four commentaries on the main collections as his and debate the rest.
The Mahāvihāra won. King Parākramabāhu I, who ruled from 1153 to 1186, brought the island's monks under its leadership, and monks from Southeast Asia came to Sri Lanka for its ordination. Since at least the twelfth century, scholars hold, Buddhaghosa's readings have been the Theravāda's orthodox understanding of its scriptures. In the twentieth century the Visuddhimagga became a main source of the insight-meditation movement, and some modern monks, Bhikkhu Sujato and Thanissaro Bhikkhu among them, argue that its account of concentration departs from the discourses it explains.