The monks of Bec, a Benedictine abbey in Normandy, wanted their prior's teaching about God in writing, and they set the terms: nothing was to be proved from the authority of Scripture, the arguments were to be plain, and even the objections of simple people had to be answered. Anselm refused for a long time, then wrote the Monologion in the mid-1070s. Those conditions became the project of his life. A Christian already believes that God exists and that God became man; Anselm held that the believer should go on to understand why these things must be so, by reason, in steps an unbeliever could follow. His next book first bore the title Faith Seeking Understanding, and scholars still use the phrase to name his method.
He had come to Bec for its teacher. At twenty-three Anselm left Aosta, crossed the Alps, and after three years in Burgundy and France reached Normandy in 1059, drawn by Lanfranc, a scholar from Pavia whose school at Bec was known above all for dialectic, the art of logical argument. Anselm entered the abbey as a novice in 1060 and became prior in 1063, when Lanfranc left to be abbot of Caen. Lanfranc, by then Archbishop of Canterbury, read the Monologion and objected that it argued without citing the Church Fathers, Augustine above all. Anselm answered him in a letter and kept the method.
The Monologion had needed a long chain of arguments, and Anselm wanted a single one that would need nothing but itself. In the preface to the Proslogion, composed in 1077 and 1078, he says the search gave him no rest; he tried to put it out of his mind, and then one day it came to him. The argument starts from a definition that even the man who denies God can understand: God is a being so great that nothing greater can be thought. Such a being exists at least in the mind of anyone who understands the words. But a being that also existed in reality would be greater still, so the greatest conceivable being cannot exist in the mind alone. God exists — and, Anselm went on, cannot even be thought not to exist.
Gaunilo, a monk of Marmoutier near Tours, replied with an island that no one can find but that is richer than every other land: by the same reasoning, he said, it would have to exist. Anselm answered that the reasoning applied to one being only, and asked that the objection and his reply be copied with the book. Thomas Aquinas rejected the argument in the thirteenth century; René Descartes built his own version in the seventeenth. Immanuel Kant objected that existence adds nothing to the idea of a thing, and classed proofs of this kind as ontological, the name the argument still carries. In the twentieth century Norman Malcolm and Alvin Plantinga rebuilt it with the logic of what is possible and what is necessary, and philosophers still dispute whether Gaunilo's island defeats it.
After Lanfranc died in 1089, King William II, known as William Rufus, kept Canterbury vacant for four years and took its revenues. In March 1093 the king fell ill at Gloucester, believed he was dying, and named Anselm. Eadmer, the Canterbury monk who became Anselm's companion and biographer, reports that Anselm refused and that the bishops forced the pastoral staff into his hand. One question dominated his fifteen years as archbishop: whether king or pope governed the church in England. In 1097 Anselm went to Rome without the king's permission and was not allowed back.
In exile in 1098 he finished Why God Became Man, which applies to the cross itself the terms the monks of Bec had set. Setting aside everything known of Christ, it argues by reason alone that sin takes from God the honour owed to him. Humanity owes the debt, but only God has anything great enough to pay it, so only someone who is both God and man can pay it. The book broke with the older view that Christ's death paid a ransom to the devil. Its account, called the satisfaction theory, passed through Aquinas into Western teaching on the atonement; John Calvin's teaching that Christ bore the punishment for sin is usually traced back to it.
King Henry I recalled Anselm in 1100 but claimed the same rights over bishops, and a second exile followed from 1103 to 1107. After the king gave up handing new bishops the ring and staff of their office in 1107, Anselm finished De Concordia, which argues that God's foreknowledge, predestination and grace leave room for free human choice. He died on 21 April 1109, leaving unfinished a dialogue on possibility, necessity and freedom, of which only fragments survive.