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Occam's razor is a minimalist principle that originated in the medieval philosophical school Nominalism. The principle states that one should never make more assumptions or assume more causes than the minimum necessary to solve a problem or find a cause for something. It is named after the 14th century English philosopher and theologian William of Occam (or Ockham).
Although the principle bears his name, William of Occam did not invent it. The concept is found in the writings of many other medieval philosophers, including Duns Scotus, Thomas Aquinas, and Durand de Saint-Pourcain, and has roots as far back as Aristotle. Willliam of Occam was one of its most prolific exponents, and he became involved in a long theological conflict with Pope John XXII because of it, therefore Occam's 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 unicorns.
 Common misuse
Woo-meisters and creationists sometimes use Occam's razor incorrectly as a handwave to avoid facing 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 assume 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. The validity of empirical observation is about as conservative as assumptions get, making induction from it ipso facto a more parsimonious argument.
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. You might cut off your fingers.
 An answer to Occam's razor
“”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)
 Occam's duct tape
The opposite mental process to Occam's razor — to avoid simplicity, to leave no entity unmultiplied and to make as many unnecessary assumptions as possible when pondering an idea — is sometimes referred to (in jest) as Occam's duct tape.
 Crabtree's bludgeon
“”No set of mutually inconsistent observations can exist for which some human intellect cannot conceive a coherent explanation, however complicated.
 Newton's flaming laser sword
Newton's flaming laser sword is another sort of razor devised by the Australian mathematician and philosopher Mike Alder. It represents a sort of logical positivism that is a common thread among the sciences. It states: "What cannot be settled by experiment is not worth debating."
Alder characterizes the flaming laser sword as "much sharper and more dangerous than Occam's razor."
 See also
- Beg the question
- Hanlon's razor
- Pierre D'Ailly
- Conspiracy theories
- Extraordinary claims require extraordinary evidence