Occam's razor (also spelt Ockham's razor) is a principle attributed to the 14th-century English logician and Franciscan friar William of Ockham (Guilhelmi Ockam and Guillermi de Ockam in Latin). Originally a tenet of the reductionist philosophy of nominalism, it is more often taken today as a heuristic maxim that advises economy, parsimony, or simplicity in scientific theories.

Occam's razor states that the explanation of any phenomenon should make as few assumptions as possible, eliminating, or "shaving off", those that make no difference in the observable predictions of the explanatory hypothesis or theory. In short, when given two equally valid explanations for a phenomenon, one should embrace the less complicated formulation. The principle is often expressed in Latin as the lex parsimoniae (law of succinctness):

entia non sunt multiplicanda praeter necessitatem (entities should not be multiplied beyond necessity).

Furthermore, when multiple competing theories have equal predictive powers, the principle recommends selecting those that introduce the fewest assumptions and postulate the fewest hypothetical entities. It is in this sense that Occam's razor is usually understood.

Further reading

Modern version:

KISS (Keep It Simple, Stupid)

However, we now know that the universe is more complicated than we can possibly imagine, and there is no evidence that simple accounts are more likely to be true than complex ones.