non sunt multiplicanda entia praeter necessitatem
(entities are not to be multiplied beyond necessity)
This principle was stated by William of Ockham (1285-1347/49). It is also called the 'Law of Economy' and the 'Law of Parsimony'.
Nevertheless, the principle was invoked before Ockham by Durand de Saint-Pourçain, a French Dominican theologian and philosopher of dubious orthodoxy, who used it to explain that abstraction is the apprehension of some real entity, such as an Aristotelian cognitive species, an active intellect, or a disposition, all of which he spurned as unnecessary. Likewise, in science, Nicole d'Oresme, a 14th-century French physicist, invoked the law of economy, as did Galileo later, in defending the simplest hypothesis of the heavens. Other later scientists stated similar simplifying laws and principles.
Ockham, however, mentioned the principle so frequently and employed it so sharply that it was called "Ockham's razor." He used it, for instance, to dispense with relations, which he held to be nothing distinct from their foundation in things; with efficient causality, which he tended to view merely as regular succession; with motion, which is merely the reappearance of a thing in a different place; with psychological powers distinct for each mode of sense; and with the presence of ideas in the mind of the Creator, which are merely the creatures themselves.