Antoine Arnauld, (1612 - August 8, 1694) — le grand as contemporaries called him, to distinguish him from his father — was a French Roman Catholic theologian and writer.

The twentieth and youngest child of the original Antoine Arnauld, he was originally intended for the bar, but decided instead to study theology at the Sorbonne. Here he was brilliantly successful, and his career was flourishing when he came under the influence of Vergier, and was drawn in the direction of Jansenism. His book, De la fréquente Communion (1643), was an important step in making the aims and ideals of this movement intelligible to the general public. Its appearance attracted controversy, and Arnauld was forced to go into hiding; for more than twenty years he dared not appear publicly in Paris.

During this time he produced innumerable Jansenist pamphlets. In 1655 two very outspoken Lettres a duc et pair on Jesuit methods in the confessional brought a motion to expel him from the Sorbonne. This motion was the immediate cause of Blaise Pascal's Lettres Provinciales . Pascal, however, failed to save his friend; in February 1656 Arnauld was ceremonially degraded. Twelve years later the so-called "peace" of Pope Clement IX put an end to his troubles; he was graciously received by Louis XIV, and treated almost as a popular hero.

He now set to work with Pierre Nicole on a great work against the Calvinists: La perpetuite de la foi de l'Eglise catholique: touchant l'eucharistie. Ten years later, however, persecution resumed. Arnauld was compelled to leave France for the Netherlands, finally settling down at Brussels. Here the last sixteen years of his life were spent in incessant controversy with Jesuits, Calvinists and heretics of all kinds.

His inexhaustible energy is best expressed by his famous reply to Nicole, who complained of feeling tired. "Tired!" echoed Arnauld, "when you have all eternity to rest in?" His energy was not exhausted by purely theological questions. He was one of the first to adopt the philosophy of René Descartes, though with certain orthodox reservations; and between 1683 and 1685 he had a long battle with Nicolas Malebranche on the relation of theology to metaphysics. On the whole, public opinion leant to Arnauld's side. When Malebranche complained that his adversary had misunderstood him, Boileau silenced him with the question: "My dear sir, whom do you expect to understand you, if M. Arnauld does not?" And popular record for Arnauld's penetration was much increased in his An de penser, commonly known as the Port-Royal Logic, which kept its place as an elementary text-book until the 20th century.

Arnauld came to be regarded as important among the mathematicians of his time; one critic described him as the Euclid of the 17th century. After his death, his reputation began to wane. Contemporaries admired him as a master of intricate reasoning; on this, Jacques-Bénigne Bossuet, the greatest theologian of the age, agreed with Henry François d'Aguesseau , the greatest lawyer. However, his eagerness to win every argument endeared him to no one. "In spite of myself," Arnauld once said regretfully, "my books are seldom very short." If not for his connection with Pascal, Arnauld's name would be almost forgotten--or, at most, live only in the famous epitaph Boileau consecrated to his memory--as

"Au pied de cet autel de structure grossière
Gît sans pompe, enfermé dans une vile bière,
Le plus savant mortel qui jamais ait écrit ;
...

Antoine Arnauld's complete works (thirty-seven volumes in forty-two parts) were published in Paris, 1775-1781. There is a study of his philosophy in Bouillier , Histoire de la philosophie cartésienne (Paris, 1868); and his mathematical achievements are discussed by Franz Bopp in the 14th volume of the Abhandilgen zur Geschichte der mathematischen Wissenschaften (Leipzig, 1902).

Arnauld conducted a extensive correspondence with Gottfried Wilhelm Leibniz, regarding the latter's "Discourse on Metaphysics".