What is Occam's razor?

If you're contemplating a complex problem, think of Occam's razor, as simplicity is often best. (Image credit: Laurence Dutton via Getty Images)

Occam's razor (also spelled Ockham's razor) cuts through complexity with a no-nonsense approach. The philosophical maxim "Numquam ponenda est pluralitas sine necessitate," written by 14th-century Franciscan friar William of Ockham, translates to "Plurality must never be posited without necessity." In other words, all else being equal, simplicity is best. 

So is this actually true? Is the simplest explanation usually the best one? 

Not exactly. Ockham never said complexity is inherently inferior to simplicity, nor did he declare complex explanations inherently wrong. Complex scientific questions often demand complex answers, and that's not at odds with Occam's razor. The principle merely states that unnecessary complexity is, well, unnecessary. 

"Occam's razor is about finding the simplest solution that works," Johnjoe McFadden (opens in new tab), a professor at the University of Surrey in the U.K. and author of the book "Life Is Simple: How Occam's Razor Set Science Free and Shapes the Universe (opens in new tab)" (Basic Books, 2021), told Live Science in an email. "It never fails so long as you remember the necessity clause."

Ockham was not the first (opens in new tab) to promote simplicity. Aristotle held that "the more limited, if adequate, is always preferable," and Ptolemy considered it best "to explain phenomena by the simplest hypothesis possible." Some three centuries after the genesis of Occam's razor, Isaac Newton would declare that "we are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances." About 200 years after that, Albert Einstein would agree that "everything should be made as simple as possible, but not simpler" (which is, in fact, a simplification of his original quote (opens in new tab)).

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When used correctly, Occam's razor works. If two computer programs accomplish the same task, the one with less code is inevitably more efficient. The simplest medical diagnosis is usually correct; hospital interns are often taught to think of horses, not zebras, when they hear hoofbeats. One implication of the second law of thermodynamics (disorder increases for any spontaneous process) is that such processes always use the least possible energy.

"Copernicus came up with the heliocentric model of the solar system solely on the basis that it was simpler," McFadden said. "The existence of a single Higgs boson was the simplest solution to the equations of particle physics. Between these points are a thousand scientific advances that depended on simplicity."

When misused, however, Occam's razor can become a blunt instrument of overgeneralization. The principle does not mean, for instance, that we blindly follow the simplest theory, whether right or wrong. "Very often the simplest hypothesis is too simple," Elliott Sober (opens in new tab), a professor of philosophy at the University of Wisconsin-Madison and author of the book "Ockham's Razors: A User's Manual" (opens in new tab) (Cambridge University Press, 2015), told Live Science in an email. "The simplicity of a hypothesis is one consideration, among others that are relevant to assessing whether a hypothesis is true."

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When it comes to data science, Occam's razor may cause more problems than it solves. In this case, "The simplest approach is usually wrong," said Pedro Domingos (opens in new tab), professor emeritus of computer science and engineering at the University of Washington in Seattle. When Domingos studied the applicability of Occam's razor to machine learning in the early 2000s, he found that a simpler (opens in new tab) model (opens in new tab) is superior to a complex one only if it is just as good at predicting new data. 

"As modern machine learning has shown over and over again — in model ensembles, deep learning, et cetera — it's usually the most complex approach that's right, Domingos told Live Science in an email. "And that's not surprising; the phenomena we're modeling are almost always more complex than the models, and the closer to their true complexity we can get, the more accurate the models."

Occam's razor nonetheless remains a useful tool for trimming the fat off of bulky assumptions, at least in our day-to-day lives. "The universe is a complicated place, but it's sometimes made more complicated through the invention of complicated explanations that suit a particular ideology, philosophy or political persuasion," McFadden said. "Occam's razor tells you to forget about all of those."