Occam’s razor is a logical and philosophical principle stated by the medieval scholar William of Ockham (1285–1347/49). It gives precedence to simplicity; that is to say, of two or more competing theories, the simpler explanation of an entity is to be preferred. The principle is also expressed as “Entities are not to be multiplied beyond necessity.”
In other words, Ockham used the principle to dispense with relations, which he held to be nothing distinct from their foundation in things. According to Ockham:
pluralitas non est ponenda sine necessitate
“Plurality should not be posited without necessity.”
Explanations can become needlessly complex. It could become coherent to add the involvement of say, leprechauns to any explanation, but Occam’s Razor would prevent such additions, unless they were causally necessary.
“The simplest hypothesis proposed as an explanation of phenomena is more likely to be the true one than is any other available hypothesis, that its predictions are more likely to be true than those of any other available hypothesis, and that it is an ultimate a priori epistemic principle that simplicity is evidence for truth.” – Richard Swinburne
Consider the following example: Two trees have fallen down during a windy night. There could be two possible explanations to account for the fallen trees:
- The wind has blown them down.
- Two meteorites have each taken one tree down, and after that hit each other and removed any trace of themselves – that, or those pesky leprechauns again.
Knowledge Guild 
One would expect a medieval scholar to be heavily bearded, but William of Ockham was in fact quite clean shaven, proving that he did, indeed, possess a razor.
Hundreds of years later, Robert J. Hanlon came up with his own, known as Hanlon’s Razor, which stated, “Never attribute to malice that which is adequately explained by stupidity.”
Science fiction writer Robert A. Heinlein said something similar years earlier, in a 1941 short story “Logic of Empire”: “You have attributed conditions to villainy that simply result from stupidity”
But the concept can be traced even further back, to Jane West’s The Loyalists (1812): “Let us not attribute to malice and cruelty what may be referred to less criminal motives. Do we not often afflict others undesignedly, and, from mere carelessness, neglect to relieve distress?”
And further back still, to Goethe’s The Sorrows of Young Werther (1774): “…misunderstandings and neglect create more confusion in this world than trickery and malice. At any rate, the last two are certainly much less frequent.”
But Peter W. Singer, in his 2009 book Wired for War, attributed the Razor to Einstein, quoting him as having said, “Never attribute to malice that which can be adequately explained by stupidity, but never rule out malice.“