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Occam's razor

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Stained-glass window showing William of Ockham
William of Ockham

Occam's razor (or Ockham's razor) is a principle from philosophy. Suppose there exist two explanations for an occurrence. In this case the simpler one is usually better. Another way of saying it is that the more assumptions you have to make, the more unlikely an explanation is. Occam's razor applies especially in the philosophy of science, but also more generally.


William of Ockham, a Franciscan friar who studied logic in the 14th century, first made this principle well known.[1] In Latin it is sometimes called lex parsimoniae, or "the law of briefness". William of Ockham supposedly (see below) wrote it in Latin as:

  • Entia non sunt multiplicanda praeter necessitatem.[1]

This translates literally as:

  • More things should not be used than are necessary.

This means that if there are several possible ways that something might have happened, the way that uses the fewest guesses is probably the right one. However, Occam's razor only applies when the simple explanation and complex explanation both work equally well. If a more complex explanation does a better job than a simpler one, then you should use the complex one.

Occam's razor is a principle, not an actual razor: the word 'razor' is a metaphor. Occam's razor gets rid of unnecessary explanations just like a razor shaves off extra hair.[2]

Further ideas

A problem with Occam's razor is that the sentence is not really about things (entia = entities), but about explanations or hypotheses. So other thinkers have come up with other versions:

  • "We consider it a good principle to explain the phenomena by the simplest hypothesis possible." Ptolemy.[3] Not only is Ptolemy earlier than Occam,[4] but Occam's supposed wording cannot be found in any of his existing works.[5]
  • "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". Isaac Newton.[6]
  • "Whenever possible, substitute constructions out of known entities for inferences to unknown entities". Bertrand Russell.[7]

In science, Occam's razor is used as a heuristic (general guiding rule or an observation) to guide scientists.[8][9][10][11][12][13]


Example: Two trees have fallen down during a windy night. Think about these two possible explanations:

  1. The wind has blown them down.
  2. Two meteorites have each taken one tree down, and after that hit each other and removed any trace of themselves.[14]

Even though both are possible, several other unlikely things would also need to happen for the meteorites to have knocked the trees down (they would have to hit each other and also not leave any marks). In addition, meteorites are fairly rare. Since this second explanation needs several assumptions to all be true, it is probably the wrong answer. Occam's razor tells us that the wind blew the trees down, because that is the simplest answer and therefore probably the right one.

Occam's razor also comes up in medicine. When there are many explanations for symptoms, the simplest diagnosis is the one to test first. If a child has a runny nose, it probably has the common cold rather than a rare birth defect. Medical students are often told, "When you hear hoof beats, think horses, not zebras".[15]

Other pages


  1. 1.0 1.1 "Ockham’s razor". Encyclopædia Britannica. Encyclopædia Britannica Online. 2010. Retrieved 12 June 2010.
  2. Anu Garg (17 May 2010). "A.Word.A.Day - Ockham's razor". Retrieved 30 October 2011.
  3. Franklin, James (2001). The science of conjecture: evidence and probability before Pascal. The Johns Hopkins University Press., 241.
  4. Ptolemy was a Greek who (probably) lived and worked in Alexandria, from about 85 to 165 AD. He is famous for his work on astronomy and geography.
  5. Crombie A.C. 1959. Medieval and early modern philosophy. Cambridge, MA: Harvard, vol 2, 30
  6. Hawking (2003). On the shoulders of giants. Running Press. p. 731. ISBN 0-7624-1698-X .
  7. Stanford Encyclopedia of Philosophy, 'Logical construction'
  8. Hugh G. Gauch 2003. Scientific method in practice. Cambridge University Press. ISBN 0-521-01708-4, ISBN 978-0-521-01708-4
  9. Hoffmann, Roald et al 1997. Ockham's Razor and chemistry. Journal for Philosophy of Chemistry. 3, 3–28.
  10. Alan Baker (2004, 2010), "Simplicity", Stanford Encyclopedia of Philosophy, California: Stanford University, ISSN 1095-5054 ,, retrieved 25 July 2012
  11. Courtney A & M (2008), "Comments regarding "On the Nature Of Science"", Physics in Canada 64 (3): 7-8,, retrieved 1 August 2012
  12. Gernert, Dieter 2007. Ockham's Razor and its improper use. Journal of Scientific Exploration. 21, 135–140.
  13. Elliott Sober 1994. Let's razor Occam's Razor. In Dudley Knowles (ed) Explanation and its limits. Cambridge University Press, 73-93.
  14. Singh, Simon (2004). Big Bang: The Origin of the Universe. New York, NY: HarperCollins Publishers. p. 45. ISBN 0-00-716221-9 .
  15. Sotos, John G. (2006) [1991]. Zebra Cards: an aid to obscure diagnoses. Mt. Vernon, VA: Mt. Vernon Book Systems. p. 1. ISBN 9780981819303 .