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Occam's Razor

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"When I see a crop circle, I could hypothesize a conspiracy of local farmboys who spent a whole night in a field trampling crops down with primitive earthling tools. But a much simpler hypothesis would be a fellow alien with a crop-writer who wanted to contact a ship in orbit. Hitchhikers do this all the time."

Also called:

  • Law of parsimony
  • Law of economy
  • Law of succinctness
  • The Lex Parsimoniae

Occam's Razor (also listed by our good friends at Rational Wiki) is an epistemological razor (a logical principle that is used in deductive reasoning to evaluate theories). It is named for the 14th-century Franciscan friar and scholastic philosopher William of Ockham, who was a particularly assiduous applier and proponent of the Law of Parsimony, although by no means the first to describe or postulate it. It is often used to evaluate the usefulness of a theory. Its main tenet is that "Entities must not be multiplied beyond necessity." It can be summed up with the phrase "When you hear hoofbeats, think horses, not unicorns."

Most theories have a foundation of underlying premises (the aforementioned "entities"), all of which need to be true for the theory itself to be true. Occam's Razor suggests believing the theory with the fewest underlying premises (the aforementioned "not multiplied beyond necessity").


Example: There have been theories that Ancient Astronauts built the Egyptian Pyramids instead of humans. For this to be true, we'd need the following givens:

  1. aliens exist note 
  2. they are intelligent note 
  3. they exist contemporaneously with humans note 
  4. they're more advanced than us note 
  5. they develop interstellar/intergalactic travel note 
  6. they know how to find Earth note 
  7. they can build pyramids note 
  8. they would not leave any evidence of their existence note 
  9. they would waste time building pyramids. note 

The more normal theory only requires that:

  1. humans exist note 
  2. humans can build pyramids note 
  3. humans would waste time building pyramids. note 

You can probably guess which theory Occam would agree with, and why.

In short, when trying to examine an incident to figure out why it happened, a simple answer involving the commonplace and reasonable is more likely to be correct. Note: more likely, not always. For decades, doctors presumed that people got stomach ulcers because of stress and bad diet, as this was the simplest explanation at the time; thus they encouraged people to reduce the conditions that caused them to be exposed to stress and to eat a bland diet. Then it was discovered that bacteria were the true cause of most stomach ulcers, meaning stress and diet had little to do with whether or not you got an ulcer. The previous treatments, though reasonable, were also wrong.

Occam's Razor is the bane of Conspiracy Theorists everywhere since conspiracies usually rest on a lot of shaky assumptions. For example, the Apollo moon landings, which a good percentage (in the single figures) believe was hoaxed. Often people will find "evidence" that the landings could never have taken place, but it rests on the arguments that the US government:

  1. were willing and able to expend untold billions of dollars more than they were ever officially allocated on smoke-and-mirrors attempts;
  2. were smart enough to fool 99% of the population (which some would contest);
  3. were simultaneously stupid enough not to cover their tracks;
  4. had the technological and film-making ability to actually fake the moon-landing footagenote ;
  5. were able to pay off and swear to silence thousands of people working at NASA and other companies for forty years when they couldn't even pull off a simple burglary;
  6. were able to pay off and swear to silence the Soviet Military (and its intelligence directorate, the GRU) during a period where the Red Army, its budget, its personnel, and its material was subjected to intense and ongoing analysis by its enemies (the Communist Party and the KGB) without arousing their suspicions, or paid off and swore to silence all three factions despite the natural inclination of all three to seriously humiliate their mutual enemy;
  7. and either persuaded the Soviets to destroy every record of the deal so thoroughly that no trace of it remains in now-declassified Soviet archives, even though the Soviets had never bothered to do anything of the sort before (on the grounds that their country would endure until the end of human history and said archives would never be seen by hostile eyes), or have since bought the silence of The New Russia as well.

After that, you'd think that the simplest explanation was to, you know, actually send people there (That Mitchell and Webb Look has a brilliant series of sketches on this idea, including the moon landing).

The Razor is commonly misinterpreted as saying, "The simplest theory is the best." or, even worse, "The simplest theory is always right." This is not correct in Real Life unless it is the simpler of two theories which make predictions with identical degrees of accuracy. All other aspects of the theory have to be equal before simplicity is taken into account. It also requires that all the data are accounted for. Newtonian physics are simpler than modern theories and were sufficient to take man to the Moon, but (with all due respect to the man) Sir Isaac simply could not explain all the data eventually collected—especially since a lot of the offending material had not been collected when Principia Mathematica was published. This required some other smart man—namely, Albert Einstein—to formulate more complex theories, particularly "Relativity"note . It should, however, be noted that since Einsteinian physics make very little difference to results at macroscopic scales or with objects travelling at non-relativistic speeds (and often the difference they do make is so small as to amount to false precision based on the initial variables), the Razor would still support using the Newtonian equations for such calculations, which is why we do so.

Another very common mistake is to summon up the Razor in a debate over a point that is entirely moot in order to add weight to a particular argument. This usage is entirely fallacious as the Razor does nothing more than recommend the hypothesis that makes the fewest new assumptions. It is not a magical tool that points to the right answer. In a lab, it will be used hundreds or thousands of times, with each and every one of the chosen hypothesis being rigorously tested, before a correct answer is found. In a debate, the Razor will be used once and will, invariably, choose the user's answer as the 'right' one. Funny, that. Another problem thrown up in such situations is the scramble to determine whose theory is simplest and thus which one "benefits" from the application of the Razor. Unfortunately, thinking that Occam's Razor is a magic tool for finding the right answer is not restricted to online debates, it is also an altogether too common reason for medical misdiagnosis. While not every condition is worthy of Dr. House (who actually declared Occam's Razor to be a fallacy; the whole point of his department was to handle weird cases where the simple answer already failed), doctors have something of a tendency to default to the most likely diagnosis and may ignore evidence to the contrary, particularly if they are tired or busy. This means that doctors sometimes refuse to diagnose, or even look for, diseases such as meningitis, sometimes with deadly results.

Always remember: Occam's Razor is a guideline, not a rule. Be careful of facts that are subjective in nature or may not be fully established.

The inverse of this is Arkham's Razor, where the most bizarre solution is most likely to be the correct one.


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