Occam's razor; Ockhams Messer; Lex parsimoniae

Occam's razor (or Ockham's razor)^[1] often expressed in Latin as the lex parsimoniae, translating to law of parsimony, law of economy or law of succinctness, is a principle that generally recommends, when faced with competing hypotheses that are equal in other respects, selecting the one that makes the fewest new assumptions.

William of Ockham (c. 1285–1349) is remembered as an influential nominalist but his popular fame as a great logician rests chiefly on the maxim attributed to him and known as Ockham's razor. The term razor (the German "Ockhams Messer" translates to "Occam's knife") refers to distinguishing between two theories either by "shaving away" unnecessary assumptions or cutting apart two similar theories.

This maxim seems to represent the general tendency of Occam's philosophy, but it has not been found in any of his writings. His nearest pronouncement seems to be Numquam ponenda est pluralitas sine necessitate [Plurality must never be posited without necessity], which occurs in his theological work on the Sentences of Peter Lombard (Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombardi (ed. Lugd., 1495), i, dist. 27, qu. 2, K).

In his Summa Totius Logicae, i. 12, Ockham cites the principle of economy, Frustra fit per plura quod potest fieri per pauciora [It is futile to do with more things that which can be done with fewer].

—Thorburn, 1918, pp. 352–3; Kneale and Kneale, 1962, p. 243.