A Franciscan friar (devoted to a life of poverty and minimalism) and an early English philosopher, William of Ockham (ca. 1285-1349) wrote "Pluralitas non est ponenda sine necessitate" or "Pluralities should not be posited without necessity." It is often paraphrased as "Simple explanations are preferred to complex ones" and has become a basic principle of the Scientific method called Occam's (or Ockham's) Razor.
It is widely accepted that Ockham wrote it; it is also thought that because he used it so frequently that his name eventually was attached to it as the author. It is somewhat ironic that it is used commonly by atheists in arguing that God does not exist on the grounds that God is an unnecessary variable in our quest for the "Theory of Everything". According to their stance, everything can be explained without assuming the extra metaphysical existence of a Divine Being, so it is unnecessary to posit one.
Occam's Razor is also misinterpreted or given to the idea that "perfection is simplicity". Albert Einstein had this in mind when he wrote that "Theories should be as simple as possible, but no simpler."
Occam's razor has also been referred to as the "principle of parsimony" and the "principle of simplicity"; the common interpretion means something like "the simpler the explanation, the better" or "K.I.S.S." (keep it simple, stupid). Another proverb expressing the idea that is often heard in medical schools is "When you hear hoofbeats, think horses, not zebras."