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Occam's razor is the principle that entities must not be multiplied beyond necessity ("entia non sunt multiplicanda praeter necessitatem") and the conclusion thereof, that the simplest explanation or strategy tends to be the best one. The principle is attributed to 14th-century English logician, theologian and Franciscan friar, William of Ockham. Occam's razor may be alternatively phrased as plurality should not be posited without necessity ("pluralitas non est ponenda sine necessitate").
Occam's razor 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 (translating to the law of parsimony, law of economy or law of succinctness). When competing hypotheses are equal in other respects, the principle recommends selection of the hypothesis that introduces the fewest assumptions and postulates the fewest entities while still sufficiently answering the question. It is in this sense that Occam's razor is usually understood. To quote Isaac Newton, "We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances. Therefore, to the same natural effects we must, so far as possible, assign the same causes."
In science, Occam’s razor is used as a heuristic (rule of thumb) to guide scientists in the development of theoretical models rather than as an arbiter between published models. In the scientific method, Occam's razor is not considered an irrefutable principle of logic, and certainly not a scientific result.
Occam's razor is not an embargo against the positing of any kind of entity, or a recommendation of the simplest theory regardless of the evidence (note that simplest theory is something like "only I exist" or "nothing exists").
The other things in question are the evidence for the theory. Therefore, according to the principle, a simpler but less correct theory would not be preferred over a more complex but more correct one. It is this fact which gives the lie to the common misinterpretation of Occam's Razor that "the simplest" one is usually the correct one.
For instance, classical physics is simpler than more recent theories; nonetheless it would not be preferred over them because it is demonstrably wrong in certain respects.
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