GAUNILO. - A Benedictine monk of Marmoutiers near Tours in the second half of the 11th century. He was the son of the viscount of Tours and, before entering the monastery, held the office of treasurer of the church of St Martin in Tours.
According to annotations in some 12th-century manuscripts, he is the author of the *Liber pro insipiente*, which wittily criticises St Anselm’s *Proslogion*. The saint replied and requested that the controversy, together with the chapters discussed (2-4), always be appended to his work.
Bibl.: E. Marçine, *Histoire de Marmoutiers*, ed. U. Chevalier, I, Tours 1874, p. 363; *Opera s. Anselmi*, ed. Fr. S. Schmit, I, Edinburgh 1946, pp. 123-39.