What he wrote
His best known work is the Visuddhimagga, the Path of Purification. It sets out the whole path under three headings, virtue, concentration and wisdom, and it is practical: forty subjects of meditation, how to choose between them, what the four jhānas are and how to tell one from another, and the stages by which insight matures.
He also wrote commentaries on much of the canon, including the long and middle length discourses and the monastic code. He describes himself as translating and arranging the old Sinhala commentaries of the Mahāvihāra rather than inventing anything.
Why it mattered
Before him the tradition had an enormous canon and a mass of commentary. After him it had a single coherent account, and a vocabulary in which teachers across Sri Lanka, Burma and Siam could agree about what a term meant.
The influence reaches into the present. Modern insight meditation movements take their map of the stages of practice largely from him, often without knowing it, since the suttas themselves do not lay the stages out in that order.
What is uncertain
Almost nothing about his life can be established. The later chapters of the Mahāvaṃsa and a much later biography say he was born in India, perhaps near Bodh Gaya or in the south, came to Sri Lanka to study the commentaries at the Mahāvihāra, and wrote there. His name means voice of the Buddha, which may be a title rather than a name. Even his century rests on tradition.
Why it matters
His commentaries become foundational for Theravāda interpretation of the Pāli Canon.
History and tradition
What historians can say
The books are certain and the man is barely visible. Scholars note that although he presents himself as a transmitter, he also shapes and systematises, and at times goes beyond what the earlier texts actually say. Where he does, the Theravāda of later centuries usually followed him rather than them.
What Buddhist tradition remembers
Tradition tells of a test: the elders gave him two verses and asked him to show what he could do, and he produced the whole Visuddhimagga. The gods are said to have hidden the manuscript twice, so that he wrote it three times, and the three copies agreed word for word.