Occam’s razor, also Ockham’s razor, [1] is the principle that “entities should not be multiplied unnecessarily.” It is apocryphally attributed to 14th-century English logician and Franciscan friar, William of Ockham. The principle states that the explanation of any phenomenon should make as few assumptions as possible, eliminating those that make no difference in the observable predictions of the explanatory hypothesis or theory. The principle is often expressed in Latin as the lex parsimoniae (“law of parsimony”, “law of economy”, or “law of succinctness”): entia non sunt multiplicanda praeter necessitatem, roughly translated as “entities must not be multiplied beyond necessity.” An alternative version Pluralitas non est ponenda sine necessitate translates “plurality should not be posited without necessity.”[2]
When multiple competing hypotheses are equal in other respects, the principle recommends selecting the hypothesis that introduces the fewest assumptions and postulates the fewest entities. It is in this sense that Occam’s razor is usually understood.
To straightforwardly summarize the principle as it is most commonly understood, “The simplest explanation for a phenomenon is most likely the correct explanation.”
Originally a tenet of the reductionist philosophy of nominalism, it is more often taken today as a heuristic maxim (rule of thumb) that advises economy, parsimony, or simplicity, often or especially in scientific theories. Here the same caveat applies to confounding topicality with mere simplicity. (A superficially simple phenomenon may have a complex mechanism behind it. A simple explanation would be simplistic if it failed to capture all the essential and relevant parts. Instead, one should choose the simplest explanation that explains the most data.)