Occam's razor
From RationalWiki
Occam's Razor refers to 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 for solving the problem. It is named after the 14th century English philosopher and theologian William of Occam (or Ockham).
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.
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") to work properly.
[edit] Expressions of the principle
William of Occam often expressed it as Numquam ponenda est pluralitas sine necessitate (A plurality [of causes] should never be posited unless necessary), but the best known expression 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).
In modern terms, it has been expressed as "when you have two competing theories which make exactly the same predictions, the one that is simpler is the better."
