Ockham's razor
See Ockham's razor in American Heritage Dictionary 4
also Oc·cam's razor (ŏkˈəmz)
noun A rule in science and philosophy stating that entities should not be multiplied needlessly. This rule is interpreted to mean that the simplest of two or more competing theories is preferable and that an explanation for unknown phenomena should first be attempted in terms of what is already known. Also called law of parsimony.
