Definition · Plain-language
Occam’s razor
Occam’s razor is the principle that, when several explanations fit the evidence equally well, the simplest one — making the fewest assumptions — should be preferred.
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Prefer the fewest assumptions
Occam’s razor, attributed to the medieval philosopher William of Ockham, is a principle of parsimony: when two or more explanations account for the same evidence equally well, you should favour the one that makes the fewest assumptions or introduces the fewest new entities. It is often paraphrased as “the simplest explanation is usually the best”, though “simplest” here means fewest unsupported assumptions, not necessarily the shortest or easiest to grasp. The razor is a tool for trimming away needless complications — the metaphorical razor shaves off assumptions that do no explanatory work.
A guideline, not a proof
A vital qualification is that Occam’s razor is a heuristic — a rule of thumb for choosing where to start — not a law of nature or a proof. It does not guarantee that the simplest explanation is the true one; reality is sometimes genuinely complicated. The razor only says that, without evidence to justify the extra complexity, you should not assume it. If new evidence demands a more complex explanation, the razor offers no resistance. It works best as a tie-breaker between hypotheses that otherwise explain the data equally well.
How it is used
Occam’s razor is woven through scientific reasoning. Faced with two theories that predict the same observations, scientists prefer the one with fewer free parameters, because it is easier to test and less likely to be over-fitted to the data. In medicine, the related principle that “common things are common” steers a doctor toward a single likely diagnosis rather than several rare ones at once. In everyday life it advises that, hearing hoofbeats, you should think horses before zebras. The razor sharpens thinking by demanding that any added complexity earn its place with evidence.
Key facts
At a glance
- Definition: prefer the explanation that makes the fewest assumptions
- Also called: the principle of parsimony
- Attributed to: the medieval philosopher William of Ockham
- Meaning of “simple”: fewest unsupported assumptions, not easiest to understand
- Status: a guideline for choosing hypotheses, not a law of nature
- Limitation: does not guarantee the simplest explanation is correct
Common misconceptions
What people often get wrong
Often heard: Occam’s razor proves the simplest explanation is always true.
Actually: It is a guideline, not a proof. It says to prefer the simplest adequate explanation as a starting point, but reality is sometimes complex and evidence can demand a more involved answer.
Often heard: “Simplest” means the shortest or easiest-to-understand explanation.
Actually: It means the explanation with the fewest unsupported assumptions or entities — which is not always the one that is briefest or most intuitive.
Often heard: Occam’s razor forbids ever considering complex explanations.
Actually: It only discourages needless complexity. When evidence justifies extra assumptions, the more complex explanation is appropriate; the razor merely requires that complexity earn its keep.
Going deeper
