What Is Occam's Razor? Why the Simpler Explanation Usually Wins
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A patient with a cough, a fever and a rash could have three separate illnesses or one that causes all three, and every doctor is trained to look for the one first. That instinct has a name and a history: it is the principle that among explanations that fit the facts, the one with fewest assumptions should be preferred, attributed to a fourteenth-century Franciscan friar, and it runs through science, medicine, detective fiction and everyday argument, where it is more often invoked than understood.
The friar and the rule
William of Ockham, born in the Surrey village of that name around 1287, was a philosopher and theologian at Oxford who fell out with the Pope, fled to the court of the German emperor and died in Munich around 1347, probably of the Black Death. His razor, as it was later called, appears in his writings in several forms, the commonest being that plurality should not be posited without necessity. He used it to cut away the abstract entities that medieval philosophy had accumulated, arguing that universals such as redness or humanity are not things existing alongside red objects and humans, but names for what such objects have in common.
The principle is older than him. Aristotle preferred the demonstration from fewer postulates, and Ptolemy wrote that it is a good principle to explain phenomena by the simplest hypothesis possible. Ockham gets the credit because he used it so often and so sharply, and the word razor, first attached to it in the nineteenth century, describes the use: shaving away what is not needed.
What it claims and what it does not
The razor is a rule for choosing between explanations that account equally well for the evidence. It says nothing about which is true; it says which to work with until the evidence separates them. It does not say the simplest explanation is correct, and it does not say the universe is simple. It says that an explanation carrying an extra assumption, which does no work in accounting for the facts, has taken on a liability for no gain, since the extra assumption is one more thing that could be false. Its main uses:
- •In science: prefer the theory with fewer free parameters, since a theory that can be adjusted to fit anything predicts nothing
- •In medicine: seek one diagnosis that explains all the symptoms before assuming several coincidental conditions
- •In statistics: penalise models with more parameters, as the standard model-selection criteria explicitly do
- •In everyday reasoning: when the keys are missing, check the coat pocket before suspecting a burglar who took nothing else
Why simplicity should count
Why a simpler hypothesis deserves preference is not obvious, and philosophers have given several answers. One is probabilistic: each assumption has some chance of being wrong, so a hypothesis with more of them is, other things being equal, less likely to be entirely right. Another is practical: simpler hypotheses are easier to test and quicker to discard, so a scientist who prefers them learns faster. A third, associated with the physicist and philosopher Ernst Mach, is that theories are economical descriptions of experience and the most economical is the best by definition. None of these proves that nature is simple, and the razor is best regarded as a rule of method rather than a law of the world.
The history of science is on its side more often than not. Copernicus's sun-centred system was preferred partly because it did without the epicycles that Ptolemy's Earth-centred one needed; Newton's single gravity replaced separate accounts of falling apples and orbiting moons; and the periodic table replaced a list of elements with a pattern. Each time, the simpler theory turned out to be not only tidier but truer.
Where it goes wrong
The razor misleads when simplicity is measured badly or when the explanations do not in fact fit the evidence equally. The simplest explanation of the fossil record, in a superficial sense, is that the animals were always there; it is not the one with fewest assumptions once the evidence of geology and genetics is counted. Conspiracy theories often claim the razor for themselves, presenting a single hidden cause as simpler than a tangle of accidents, while requiring the silence of thousands of people, which is a very large assumption. And in medicine the rule has a rival called Hickam's dictum, that a patient can have as many diseases as they please, which is a reminder that in an elderly patient several conditions at once is not an improbable explanation but a common one.
Einstein is often quoted as saying that everything should be made as simple as possible, but no simpler. He seems not to have said it in those words, but the second clause is the point: the razor removes what does no work, and an explanation that has been cut back until it no longer fits the facts has been shaved too far.
The takeaway
Occam's razor is the rule that, between explanations that fit the evidence equally, the one with fewer assumptions should be preferred, because each assumption is a further way of being wrong and a theory that can be adjusted to fit anything explains nothing. Named for a fourteenth-century friar who used it to prune medieval metaphysics, it is a guide to method rather than a claim that the world is simple, and it is misused whenever the competing explanations do not actually fit the facts equally well.