In simple terms
Occam’s razor says that when two explanations account for the same evidence, the one assuming less is usually the better bet.
It is not a law of nature. It is a working preference, a way of deciding where to start when the evidence alone cannot separate two ideas.
How it works
Philosophers separate two things the razor can trim.
One is elegance, meaning the number and conciseness of a theory’s basic principles. The other is parsimony, meaning how many kinds of things the theory says exist. The two often pull in opposite directions, because adding an entity can make the principles simpler.
The medieval link is looser than the name suggests. The modern slogan about not multiplying entities beyond necessity is tied only very tenuously to William of Ockham, the 14th century English friar it is named for. Of the Latin versions usually quoted, two do appear in his own works: pluralitas non est ponenda sine necessitate, and frustra fit per plura quod potest fieri per pauciora.
Newton set out his own version as a rule of reasoning: admit no more causes of natural things than are both true and sufficient to explain what we observe.
Why it matters
The razor only bites when rival explanations predict the same observations. Einstein’s special relativity and Lorentz’s ether theory predicted the same results, so physicists kept the account that did not need an ether.
Where predictions differ, the razor has nothing to say. Testing settles it.
It is also easier to state than to apply. Simplicity is not one measurable quantity, and how simple a theory looks can depend on the language used to write it down. Philosophers still disagree about why a simpler theory should be more likely to be true.
Where you’ll see it
- Medical teaching, where students learn to think of common causes first.
- Model selection in statistics and machine learning, which penalizes extra parameters.
- Arguments about extraordinary claims, from conspiracy theories to reports of the paranormal.
- Everyday troubleshooting, where you check the plug before suspecting the circuit board.
Example
A phone will not switch on. A flat battery and a failed logic board both explain it, so Occam’s razor says charge it before opening it.
Often confused with
The idea that the simplest answer is always right. The razor ranks explanations that the evidence supports equally. It does not promise that the tidier one is true.
Key facts
- Modern formulations of the razor are connected only very tenuously to the 14th century figure William of Ockham.1
- Philosophers distinguish syntactic simplicity, or elegance, which counts a theory's basic principles, from ontological simplicity, or parsimony, which counts the kinds of entities it postulates.1
- The two kinds of simplicity typically pull in different directions, because postulating extra entities can allow a theory to be formulated more simply.1
- Newton's first rule of reasoning allows no more causes of natural things than are both true and sufficient to explain their appearances.1
- Of the three Latin phrasings commonly quoted, only pluralitas non est ponenda sine necessitate and frustra fit per plura quod potest fieri per pauciora appear in Ockham's own works.2
- The principle applies only where rival theories make the same predictions; where predictions differ, empirical testing decides.2
In the news
Quick checkWhen does Occam's razor actually help you choose between two theories?Show answer
Only when both explain the same evidence and predict the same observations. If their predictions differ, testing decides instead.
Sources
- Stanford Encyclopedia of Philosophy. Simplicity. Undated (accessed 15 September 2026)
- University of California, Riverside (Physics FAQ). What is Occam's Razor?. Undated (accessed 15 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 16, 2026Researched and drafted with AI assistanceReport an issue