Ockham's razor

William of Ockham (c. 1287 to 1347) was an English Franciscan friar who studied and taught at Oxford, and probably took his name from the village of Ockham in Surrey. Around 1323 he finished his Summa Logicae, a major handbook of logic. In it and in other writings, he held that only individual things exist. Universals such as humanity or whiteness are not real things shared by many items but concepts, or names, in the mind. This position, nominalism, attacked the realist views of Aquinas and Duns Scotus, who held that universals have some real basis in things. The famous principle attributed to him is that plurality is not to be posited without necessity, that is, a theory should not assume more things than it needs to explain the facts.¹²³

The wording most people know, that entities should not be multiplied beyond necessity, is not found in his works, and the name Ockham's razor was applied centuries later. Ockham used the principle to cut away what he saw as invented entities, such as the abstract forms that other scholastics defended. He also argued that many doctrines of theology cannot be shown by reason and rest on faith alone. In 1324 he was summoned to the papal court at Avignon to answer charges of heresy, and he also joined the Franciscan side in the dispute over apostolic poverty with Pope John XXII. In 1328 he fled with the Franciscan leader Michael of Cesena to the protection of Emperor Louis IV in Munich, and the pope excommunicated him. He died around 1347, perhaps of the Black Death.¹²³

As the rule favoring the simpler of two explanations that fit the facts, it became a working principle of science, from Newton to modern statistics and machine learning. It is a guide to choice, not a proof that the simpler theory is true.³