Occam Never Said the Famous Sentence Attributed to Him

William of Ockham, a Franciscan friar working in the early 1300s, did write a lot about parsimony. He argued, in his commentaries on Aristotle and on the Sentences of Peter Lombard, that one should not posit a plurality without necessity — pluralitas non est ponenda sine necessitate. He used the principle as a rhetorical knife against rivals who hung extra metaphysical entities off their theories.

The sharper formulation everyone quotes — entia non sunt multiplicanda praeter necessitatem, entities should not be multiplied beyond necessity — is not in his writings. Historians trace it to a 17th-century Irish Franciscan named John Punch, who attributed something like it to Ockham in 1639. The slogan stuck. The label "Ockham's razor" itself is a 19th-century coinage by William Hamilton.

What the principle actually buys you is more limited than the popular version suggests. It is not a guarantee that the simplest explanation is true. It is a tiebreaker: when two theories explain the same evidence equally well, prefer the one with fewer assumptions. The reason is practical. Each extra assumption is another place the theory can be wrong.

In statistics this shows up as the bias-variance tradeoff. Add enough parameters and you can fit any data set perfectly, including its noise. The model will then generalize badly. Information criteria like AIC and BIC penalize complexity for exactly this reason. Bayesians get the same result through marginal likelihood, where simpler models earn an automatic prior advantage.

Ockham's actual contribution was theological. He used the razor to argue against universals, against intellectual entities medieval theology had been quietly accumulating for centuries. He was excommunicated for related reasons. The razor outlived the controversies it was forged in.