French theologian and philosopher. A brother of Angelique Arnauld (see following entry), from 1638 he was in touch with Saint-Cyran and in 1641 he retired to Port-Royal. His book De la fréquente communion (1643), with its stress on the need for a thorough preparation for Communion and its emphasis on the right dispositions, did much to propagate Jansenist principles among a wide public. From 1644 he was the acknowledged leader of the Jansenists. An attack on the Jesuit method in the confessional led to his being censured by the Sorbonne in 1656 and solemnly degraded; after the ‘Peace of the Church’ of 1668, his doctorate was restored. With P. Nicole he then began a major treatise against the Calvinists, La Perpétuité de la foi catholique touchant l'Eucharistie (1669–74).