What is Occam's razor, and where does it come from?

Occam's razor is the principle that when two competing ideas explain the same phenomenon equally well, the simpler one should be preferred. It is named after William of Ockham, a 14th-century English friar, theologian and logician. The name is a later label, and similar ideas appear in writers who lived long before and after him. [1] [2]

A common Latin wording is that plurality must never be posited without necessity, often rendered as entities must not be multiplied beyond necessity. The word razor refers to shaving away unnecessary assumptions when choosing between theories. Scientists use it as a heuristic, or rule of thumb, in building models, and it is applied in fields such as biology and medical diagnosis. [2] [3]

It is a guideline, not a law. It does not demand oversimplification, and if a more complex theory explains the facts better, that theory should be preferred. The Stanford Encyclopedia of Philosophy notes that it can be read as an epistemic principle, meaning it is rational to believe the simpler theory, or as a methodological one, meaning it is rational to adopt the simpler theory as a working hypothesis. These readings need different justifications. [1] [3]

William of Ockham (c. 1285 to 1348, with sources giving slightly different dates) is the usual credit. The Stanford Encyclopedia says the sentiment is certainly his, and that his fullest version of the razor allows extra entities only when supported by reason, experience or infallible religious authority such as Scripture. It also says the familiar slogan appears nowhere in his texts. [4] [2]

Other accounts question how much the credit is deserved. The AAAS says the true origin is debatable and that Ockham gets the credit largely because of the 1852 writings of the Scottish philosopher Sir William Hamilton, who coined the term Occam's razor. The Century Dictionary, quoted by Etymonline, says Ockham did not make much use of the principle, which it attributes rather to his contemporary William Durand de St. Pourçain. The Stanford entry adds that most philosophers, medieval or otherwise, would accept the sentiment, since few favor an unnecessarily large ontology. [2] [5] [4]

Aristotle wrote that a demonstration deriving from fewer postulates is superior, other things being equal. Ptolemy favored explaining phenomena by the simplest hypothesis possible, and Aquinas wrote that it is superfluous to use several means where one suffices. Galileo and Newton accepted versions of the idea, and Newton's first Rule of Reasoning says to admit no more causes of natural things than are both true and sufficient to explain their appearances. Kant and the chemist Lavoisier also invoked it. [3] [2]

The Stanford Encyclopedia notes that modern formulations are only tenuously connected to the historical Ockham, and that contemporary philosophers tend to treat the razor as a principle of theory choice. [3]

The AAAS says the razor has sometimes obstructed progress, citing simplistic and initially incorrect explanations accepted for meteorites, ball lightning, continental drift, atomic theory and DNA as the carrier of genetic information. It adds that the razor does not simply pick the simplest theory but tries to find the best theory given the knowledge and evidence available at the time. [2]