How Occam's Razor Works

By: Shannon Cicero  | 
A black and white photograph or detective with a plaid cap, pipe and spyglass
It’s elementary. The simplest explanation is usually the correct one. Or is it? Headhunters/Getty Images
What is Occam's Razor?

­­You've probably heard it before: The simplest explanation is usually the right one. Detectives use it to deduce who's the likeliest suspect in a murder case — you know, the butler did it. Doctors ­use it to determine the illness behind a set of symptoms. Whenever you have two or more theories that make exactly the same predictions, the simpler one is always best.

This line of reasoning is called Occam's razor. It's used in a wide variety of ways throughout the world as a philosophical tool to slice through a problem or situation and eliminate unnecessary elements. But what we call the razor is a little different than what its author originally wrote. There are two theories that are considered the basis of Occam's razor, and they were originally written in Latin:


  • The Principle of Plurality: Plurality should not be posited without necessity
  • The Principle of Parsimony: It is pointless to do with more what is done with less

Taken together, they represent the basis of humanity's investigation into the universe, and the way we see our environment is largely based upon Occam's razor. There's no telling what kind of world we would live in today without Occam's razor. Would we have the Internet? Would we have inoculations? The problem solving principle has also been used to justify uncertainty in quantum mechanics. Quantum mechanics works well as a mathematical theory for predictions, but doesn't predict what will happen just what the probabilities of differing outcomes are.

Consider simple systems in nature, like viruses and plants, and their ability to carry out complex tasks such as infection and photosynthesis. We value these simple models. And when it comes to man-made systems, we tend to base structures upon what we already know works — the simplest explanation to us — like computer memory modeled on our own brain processes. All of which points to the principles of plurality and law of parsimony.

However, one of the key things that Occam's razor reveals is the subjectivity with which we view the universe. Sure the sky is blue, we know that by looking at it, but what shade of blue is it exactly? Anyone who has ever engaged in a debate over whether a dark-colored sock is black or navy can appreciate the bias of our worldview and how it affects our decisions.

In this article, we'll examine the ability of Occam's razor to become distorted, as well as who distorts it, who prizes it and who shuns it. But first, who exactly came up with this simple, yet complex idea? In the next section, we'll learn about the author of Occam's razor.


William of Occam

Portrait drawing of William of Occam.
William of Occam also known as Franciscan friar William of Ockham.

So who is this Occam fellow? Actually, Occam (or Ockham) is a town in England, not a man. More specifically, its the town where William of Occam was born. William lived from about 1285 to 1349, during the medieval age, a time when surnames were uncommon and people were known by their place of provenance [source: Beckett].

William lived as a philosopher and a Franciscan monk, a pious man who took very seriously his vow of poverty, meaning he lived using only what was absolutely necessary. One might get the impression that it was this vow of poverty — a form of simplicity — that gave William his big idea. In fact, the basis of Occam's razor was an already well-established line of medieval thought by William's time.


William captured the essence of the principle and packaged it in a way that was easily understood (by anyone who knew Latin, at least). By creating a couple of simple sentences, he managed to encapsulate a world of medieval logic, ensuring its safe passage into modern times. Kind of makes you wonder what great wisdom wasn't similarly packaged and is lost forever, doesn't it?

It's actually the Greek philosopher Aristotle to whom the idea that perfection equals simplicity and vice versa is attributed. Aristotle was known for the phrase, "The more perfect a nature is, the fewer means it requires for its operation" [source: Carroll]. Just a quick glance at the way we approach scientific investigation — and the fact that Occam's razor has survived — shows us that this idea still exists. In fact well known physicist and mathematician Isaac Newton incorporated the rule when he later said: "We are to admit no more causes of natural things than such as are both true and sufficient to explain their appearances."

While William didn't come up with the principle of parsimony, it certainly did influence the way he looked at life. Not only did William live under his minimalist vow of poverty, he wrote frequently on the subject. At one point, his order, the Franciscans, butted heads with Pope John XXII over the subject, and as is usually the case, the Pope won. William and several of his brothers were excommunicated from the church in 1328. William sought refuge in Munich, where he enjoyed protection from the sympathetic Emperor Louis IV the Bavarian, ruler of the greater Munich area at that time.

Ultimately, William prevailed: After being kicked out of the church, he wrote a convincing essay that demonstrated that Pope John XXII was a heretic — someone whose beliefs flew in the face of the church's tenets. What's more, there's an entire line of reasoning attributed to him.

So what exactly does this line of reasoning encompass? In the next section, we'll look at the far-reaching implications of Occam's razor.


Occam's Razor and the Scientific Method

Black and white photo of Albert Einstein in a cardigan.
Albert Einstein’s explanation for fluctuations in the time-space continuum was chosen based on the tenets of Occam's razor.
Toru Yamanaka /AFP/Getty Images

Occam's razor is based on the notion that simplicity equals perfection. It fits perfectly with the scientific method — the series of steps scientists take to prove or disprove something. Indeed, you could make the case that the scientific method was built upon Occam's razor.

But be careful when approaching the razor — for such a brief statement, it has an uncanny ability to be stretched or bent to fit all sorts of ideas. It's important to remember that Occam's razor proves nothing. It serves instead as a heuristic device — a guide or a suggestion — that states that when given two explanations or competing theories that make the same predictions for the same thing, the simpler one is usually the correct one. It aids scientists who are developing theoretical models.


What's implied in this principle is that simple explanations come from evidence we already know to be true, like empirical evidence — information gathered through the five senses. We know that crickets chirp because we can hear them. We know that pickles are sour because we can taste them. In this manner, things that can easily be explained using empirical evidence tend to trump explanations that are based on evidence we can't sense. It encourages us to use fewer assumptions and favor the simplest hypothesis.

Einstein and Lorentz Two Competing Theories

Here's a classic example of the use of Occam's razor. A pair of physicists — Lorentz and Einstein — both concluded mathematically that things tend to go a little wonky within the space-time continuum. For example, the closer we get to moving at the speed of light, the more we slow down.

While both arrived at the same results from their equations, Einstein and Lorentz had different explanations for them. Lorentz said that it was because of changes that take place in the "the ether." The problem is science doesn't hold that "the ether" exists — and therefore introduces a problematic element of the equation.

Einstein's explanation used no references to the ether, and therefore, his explanation eventually won out over Lorentz's. Einstein formulated his own version of Occam's razor called Einstein's Constraint. He said, "It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience."

Occam's razor gained widespread acceptance, and as a result, the principle has been expanded upon (or distorted, depending on your view) over time. The physicist Ernst Mach, for example, made the razor part and parcel with empirical evidence, when he said that scientific research should use the simplest methods to arrive at conclusions and, what's more, must exclude from that process any more evidence that isn't empirical. This is based on positivism — the idea that if something can't be proven empirically, it doesn't exist.

This kind of thinking is viewed by some as dull logic, which can result in a divide between differing ideologies. Sometimes, even both of the opposing sides use Ockham's principle to disprove each other's ideas. In the next two sections, we'll look at each of the sides. First, let's look at the people who use Occam's razor to explain their beliefs. ­


Who Uses Occam's Razor?

A religious painting of baby Jesus and Mary, surrounded by a group of seven men bowing.
Skeptics use Occam’s razor to consider everything from UFOs to religion.
Imagno/Getty Images

The example of Einstein versus Lorentz is a good illustration of who uses Occam's razor the most — scientists. To make their way through enormous equations, scientists often use the razor to get easily from point A to point B in a data set. After all, the easiest — and most times, the best — route between two points is a straight line, right?

Skeptics use Occam's razor as a fundamental tool and sometimes as evidence itself. Skeptics are people who tend to believe only what they can sense or what can be proven scientifically. This makes them foils to people who believe in conspiracy theories and religious beliefs.


But a true skeptic will tell you that he only uses Occam's razor as a tool for considering different explanations. Skeptics who truly appreciate the healthy investigation of the universe use Occam's razor to pick the simplest (and in their belief, most logical) explanation, but stop short of using it to discount other, more complex explanations. After all, evidence could come to light later on that shows the more fantastic is true, and a true skeptic's aim is to keep an open mind.

There are, however, some — skeptics and scientists alike — who wield the razor like a broadsword. To these people it proves one theory and disproves another. There are two problems with using Occam's razor as a tool to prove or disprove an explanation. One, determining whether or not something is simple (say, empirical evidence) is subjective — meaning it's up to the individual to interpret its simplicity. Two, there's no evidence that supports the notion that simplicity equals truth.

It's important to remember that the idea attributed to Aristotle says that perfection is found in simplicity is a man-made idea. It's not supported by math or physics or chemistry. And yet, it's taken by some as factual.

Debating Moral Science Can Be Subjective

Take this example. There are some creationists who say that Occam's razor proves their ideology is correct. After all, isn't it a more simple explanation to say that God created life, the universe and everything than to say it was created by a Big Bang, followed by an astounding series of interrelated coincidences?

Nice try, say evolutionists. That explanation supposes that God exists, and we have no empirical evidence that he does. This is also the case for atheists — those who don't believe in God. Atheists use Occam's razor in conjunction with Aristotle's idea of simplicity equaling perfection to prove that there is no God. If there were, say atheists, then the universe would be a whole lot simpler right?

The problem with all of these arguments is that what constitutes simplicity is subjective. What's more, we cannot rationally show that the universe could be any simpler. While we can point out redundancies on levels we can observe, we can't positively identify that these aren't necessary on the whole. Photosynthesis, for example, is a fairly complex mechanism. This doesn't mean, though, that it isn't the simplest possible means of achieving food production in a plant. We have yet to come up with a simpler process that will achieve the same end in the same system.

By now you should have a pretty good idea of the ways that Occam's razor is used to further one idea over another. In the next section, we'll take a look at people who say Occam's razor isn't necessarily a good idea and we'll find out why they oppose it.


Opposition to Occam's Razor

A photo of Lee Harvey Oswald pasted into a notebook, laying open on top of a gun.
Evidence condemning Lee Harvey Oswald. Which is the simplest theory? Was President John F. Kennedy assassinated by a lone gunman, or was his death the result of a CIA plot?
Terry Ashe/Time Life Pictures/Getty Images

You have to wonder what William would make of the use of his principle to disprove the existence of God since he was a devout Franciscan monk. He probably would point out that the razor isn't a tool that establishes proof. For this very reason, some groups say that it doesn't have much use. Others have no problem with Occam's razor, they just don't like the way other groups use it to discount theories.

Some religious thinkers don't believe the razor serves much of a purpose at all. Religion is based on faith, not evidence. The complexities of a world based on a creator god defy Occam's razor. The idea is inherently irrational, after all. What's more, we have no empirical evidence of the existence of God. But religious thinkers point out that evidence of God's existence is all around us — in the form of trees, the atmosphere and human beings.


Debunking Conspiracy Theories with Simpler Explanations

Occam's razor is sometimes used against conspiracy theorists. Usually, it's skeptics who lock horns with conspiracy theorists, using the razor as proof that conspiracists are too far-reaching in their unlikely explanations. Take, for example, the assassination of President John F. Kennedy. The idea that he was killed by a single, overzealous Communist gunman is a much simpler explanation than the idea that he was murdered by a CIA conspiracy, which would involve treachery on levels unseen in U.S. history to that point.

But does the fact that one explanation is simpler mean it's correct? Conspiracy theorists can produce all manner of circumstantial evidence that points to many different plots. But according to Occam's razor, this extra evidence would be considered irrelevant in the face of the lone gunman explanation due to unnecessary assumptions. In this case, Occam's razor only serves to fuel the debate when it's used to discount conspiracists' theories.

The limitations imposed by Occam's razor and its use with the scientific method drove one man up the wall. Charles Fort was a writer who lived in the late 19th and early 20th centuries in New York and London. He shared William of Occam's life of poverty, but for different reasons. While William took a vow of poverty for religious reasons, Fort's poverty was a byproduct of his commitment to uncovering universal truths.

Charles Fort spent his days researching in the great libraries of New York and London. There he investigated all manner of phenomena, scientifically proven and otherwise. While he admired and accepted the field of science's potential for explaining the universe, he developed a disdain for the scientific community and its stubborn refusal to accept the existence of anything that couldn't be explained by the scientific method [source: Charles Fort Institute].

To that end, Fort was determined to apply science to the investigation of the paranormal — that which exists outside of science, like ghosts and other supernatural beings. His work is carried on today, in the form of paranormal investigators at respected institutions around the world, like the University of Edinburgh. In this sense, it's easy to see the parallel between Fort's quest to apply rational thinking to explain the unseen and Occam's explorations of the nature of God.


Lots More Information

Related HowStuffWorks Articles

  • "Occam's Razor." Clavius.
  • "Occam's Razor." Principa Cybernetica Web.
  • Beckett, Dave. "Biography [of William of Occam]." University of Kent. 1994.
  • Carroll, Robert Todd. "The Skeptic's Dictionary."
  • Gibbs, Phil and Hiroshi, Sugihara. "What Is Occam's Razor?" University of California, Riverside. September 1996.
  • Hyde, Dr. Daniel C. Introduction to the Programming Language Occam.
  • Rickard, Bob. "Charles Fort: His Life and Times." Charles Fort Institute. June 7, 1997.