Occam’s Razor

I. Definition

Occam’s Razor (or Ockham’s Razor, also known as the Principle of Parsimony) is the idea that more straightforward explanations are, in general, better. That is, if you have two possible theories that fit all available evidence, the best theory is the one with fewer moving parts.

It’s important to emphasize the part about fitting all available evidence. Sometimes, the simplest explanation is very wrong because it fails to account for the evidence! In this case, Occam’s Razor does not apply. But if the explanations are both in agreement with the evidence, then Occam’s Razor tips the scale in favor of the simpler explanation.

Bear in mind, too, that the word “simple” here is used in a very specific sense. The “simpler explanation” doesn’t necessarily mean the one that’s easier to explain. Rather, it means the one with fewer moving parts: fewer variables in the equation, fewer types of abstract ideas, or fewer guesses.

That’s why the rule is also called the “Principle of Parsimony.” Parsimony means thrift or frugality, so a parsimonious person is someone who’s very careful with money. Imagine if you had to buy all the parts you needed to build your theory: each assumption, or variable would cost you $250. In that case, you’d be very careful not to buy any parts unless you really needed them — Occam’s Razor would be very important for your wallet!

Finally, remember that Occam’s Razor is a general rule of thumb, not a law. It works well enough in most cases, so scientists and philosophers rely on it in designing experiments and arguments — but you shouldn’t take it as an ironclad rule of logic.


II. Famous Quotes about Occam’s Razor

Quote 1


This is probably the most famous equation ever written. It also happens to be a fantastic example of Occam’s Razor. The equation explains the conversion rate from matter (M) into energy (E). If a certain amount of matter is destroyed, you get a certain amount of energy. But how much energy? You might expect that calculating this figure would require a vast, complex equation filling up the whole chalkboard and full of odd symbols and Greek letters (check out the equation for the Standard Model if you want to see what I mean). But it’s not! It’s a simple mathematic formula with only two variables, one well-known number (the speed of light) some multiplication, and an exponent — no calculus, no derivatives, nothing. A very parsimonious explanation of the physical world.

Quote 2

“Other things being equal, we should prefer a demonstration which derives from fewer postulates or hypotheses.” (Aristotle, Posterior Analytics)

This quote shows how old Occam’s Razor is, at least in the Western tradition. This is one of the earliest known texts on logic in the West, and one of its principles is a version of Occam’s Razor. When Aristotle says “fewer postulates or hypotheses,” he means that a proof is more convincing if it makes fewer guesses. He also says “other things being equal,” which highlights that both explanations have to fit the evidence. This, of course, is more or less how we defined Occam’s Razor — so the idea has a very long history!


III. The History and Importance of Occam’s Razor

Occam’s Razor gets its name from William of Ockham, who lived from around 1280-1350 AD in England. Ockham was a startlingly modern thinker whose ideas were far ahead of his time — his ideas on language, logic, and religion all seem to belong more to the modern era than to his own. He argued for separation of Church and State, an idea that would not come to the fore until many centuries after his death. He also believed that language was a human creation, not a divine law (which nearly everyone today agrees with, but which was highly controversial at the time).

William of Ockham was an extremely inventive and original thinker — but, ironically, the thing we remember him for was not an original idea! Occam’s Razor had been around for centuries before Ockham was ever born, as we saw in the previous section (with the quote from Aristotle). Moreover, the same idea has appeared multiple times in other traditions around the world.

For example:

Taoist thinkers have argued for a highly parsimonious view of the world — in Taoism, all aspects of the world are expressions of the single, eternal Tao, or ultimate reality. Western philosophers sometimes feel that this theory lacks explanatory power (they wonder what its implications or consequences are), but no one can doubt that it passes the test of Occam’s Razor!


IV. Examples of Occam’s Razor in Pop Culture

Example 1

In one episode of South Park, the boys visit John Edward, a man who claims he can speak with the dead. Stan thinks it’s all a trick, and he sets out to prove it. Which is the more parsimonious explanation: that magical powers exist and that John has them, but no one else does? Or that magical powers don’t exist, and John has just learned a few psychological stage tricks?

Example 2

The TV show Ancient Aliens is a constant parade of logical fallacies and factual errors. One of the most common errors made on the show is that its theories violate Occam’s Razor. For example, which of these explanations would Occam have preferred?

Humans Built the PyramidsAliens Built the Pyramids

There are humans.

Humans frequently build massive tombs.

The pyramids are one example of this behavior.

There is alien life.

Alien life has the technology to get to Earth.

Aliens are motivated to build pyramids.

Aliens left no unambiguous signs of their presence.

No scientists have discovered evidence for aliens.

Aliens covered their tracks by making the pyramids look like they were built by humans.


The first explanation has fewer moving parts (fewer “postulates,” Aristotle would say), and in addition each moving part is more straightforward. In the second explanation, you have a lot of different postulates, and each one seems to raise more questions than it answers!


V. Controversies

In some philosophical debates, both sides believe that they are on the side of Occam’s Razor. That’s because people can’t always agree on which explanation is simpler!

Example: Atheists vs. Theists (Believers)

Atheists often claim that a universe without a god has fewer moving parts, and therefore that Occam’s Razor supports their side of the debate. They point out that a theistic universe has rocks, trees, flowers, etc., plus God, whereas an atheistic universe has all the same objects minus one: God.

Believers, on the other hand, argue that the universe is more parsimonious if there is a god! They argue that atheists have to come up with complex arguments to explain consciousness, morality, and beauty, and therefore that Occam’s Razor would support believing in God. Regardless of whether you believe in God or not (that’s a separate question), which argument do you think has the support of Occam’s Razor?


The “simpler explanation” is the one that…





“Principle of Parsimony” could be re-phrased as…





Occam’s Razor is found in…





Occam’s razor is named after…






  1. Reply

    this is the best article on occams razor i have found online, due to the fact that it points out the inappropriateness of occams razor as a tool in cases where the evidencial merit is not equal in each argument.
    most websites ignore this very important condition.

  2. Reply

    I have heard of Occams Razor, but this article did a much better job of explaining it than I have seen before.

Leave a Reply

Your email address will not be published.

You may use these <abbr title="HyperText Markup Language">HTML</abbr> tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>