William of Ockham never wrote the famous line named after his razor.
'Entities must not be multiplied beyond necessity' is the classic wording of Occam's razor. The 14th-century friar it's named after never used those words, and the idea is far older than him. The line was coined by an Irish friar in 1639, and the name 'razor' came later still.
Occam's razor is the rule of thumb that, when two explanations account for the facts equally well, you should prefer the one that needs fewer assumptions. It is named after William of Ockham, an English Franciscan friar and logician who died in 1347. He stated the principle in several forms, such as 'plurality must never be posited without necessity', and applied it so often and so effectively that it became attached to his name.
But he did not invent it. Aristotle praised demonstrations that rest on fewer postulates, and Ptolemy recommended explaining phenomena with the simplest hypothesis possible. Robert Grosseteste, Maimonides, Thomas Aquinas and Duns Scotus all used versions of the idea, and similar phrases were commonplace in 13th-century scholastic writing. The best-known Latin wording, entia non sunt multiplicanda praeter necessitatem, comes from the Irish Franciscan John Punch in 1639, who called it a common axiom. The phrase 'Occam's razor' itself does not appear until centuries after Ockham's death; the theologian Libert Froidmont claimed in 1649 to have coined it.
The 'razor' image refers to shaving away unnecessary assumptions. Isaac Newton gave his own version as a rule of science: admit no more causes than are true and sufficient to explain what we see. Bertrand Russell offered another, advising that wherever possible we should build from known entities rather than inferring unknown ones.
An important nuance is often lost. The razor is a guide for choosing between hypotheses that make the same predictions, not a law that the simplest story is always true. In science it serves as a heuristic for building models, not as a final judge between them. Ockham himself exempted God's power and miracles from it. Simplicity is a sensible place to start looking, not proof that you have found the answer.
Source: Wikipedia — Occam's razor · Text summarised from Wikipedia (CC BY-SA 4.0)
