Occam's razor

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Occam's razor is a minimalist principle that originates in the medieval philosophical school Nominalism. The principle states that when answering a problem or searching for a cause for something, one should never make more assumptions or posit more causes than the minimum necessary to solve the problem. It is named after the 14th century English philosopher and theologian William of Occam (or Ockham).

The following fact cannot be overstated: Occam's razor is a heuristic, not a proof.



Although the principle bears his name, it was not invented by William of Occam. It is found in the writings of many other medieval philosophers, including Duns Scotus, Thomas Aquinas, and Durand de Saint-Pourcain, and it has roots as far back as Aristotle. However, since Willliam of Occam was one of its most prolific exponents, and since he became involved in a long theological conflict with Pope John XXII because of it, his name has been associated with the principle.

Expressions of the principle

William of Occam often expressed the principle as Numquam ponenda est pluralitas sine necessitate (A plurality [of causes] should never be posited unless necessary), but the best known version is a paraphrase by the 17th century scholar John Ponce: Entia non sunt multiplicanda praeter necessitatem (Entities are not to be multiplied beyond what is necessary).

Its scientific application is to select priority between developing theories, although it's by no means a hard and fast rule. The "simpler" theory with fewer (or less onerous) assumptions is probably the most appropriate one. For example, if you see hoof-prints on your local walking trail, think horses, not zebras. Unless your local walking trail is somewhere in Africa, then you might want to consider zebras.

Common misuse

It is sometimes used fallaciously as a handwave by woo-meisters and creationists to skirt the fact that the point they are arguing requires a completely unknown mechanism (paranormal powers, Goddidit, Aliensdidit, etc) to work properly. This claim states that Occam's Razor prefers to posit that a god (or similar device) does exist and it's simpler to believe Goddidit than that natural selection and evolution could create the complexity of life. This ignores the incredible complexity such an entity would require. It's also quite easy for skeptics to fall for this one citing Occam's razor as evidence itself, rather than finding evidence to refute a concept.

Occam's Razor is a rule-of-thumb. It is a heuristic guide for hypothesis testing. It is not:

  • Irrefutable proof of any argument that seems simpler than others.
  • A substitute for logic in any argument you make.
  • A substitute for actual scientific results.

In fact, it's probably best to avoid ever citing Occam's razor unless you're damn well sure you know what you're doing with it.

An answer to Occam's Razor

In his Summa Theologica, St. Thomas Aquinas quoted an argument for the non-existence of God based on the principle of Occam's Razor, and answered it in the following fashion:

Objection 2. Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God's existence. On the contrary, It is said in the person of God: "I am Who am." (Exodus 3:14)[1]


The opposite action to Occam's Razor — to leave no entity unmultiplied and make as many assumptions as possible when pondering an idea — is sometimes referred to in jest as "Occam's Duct Tape."

See also


  1. Summa Theologica FP Q2 A5
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