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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."''
-->-- [[https://www.youtube.com/watch?v=KMyb6tu4S_I No Edge 2: The Curvature of the Universe]]

Also called:
* Law of parsimony
* Law of economy
* Law of succinctness
* The Lex Parsimoniae

[[http://en.wikipedia.org/wiki/Occam%27s_razor Occam's Razor]] ([[https://rationalwiki.org/wiki/Occam%27s_razor also listed]] by our good friends at Wiki/RationalWiki) is an epistemological razor[[note]]a logical principle that is used in deductive reasoning to evaluate threories[[/note]] first described in the 14th century by [[http://en.wikipedia.org/wiki/William_of_Ockham William of Ockham]], an English Franciscan friar and philosopher. 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 AncientAstronauts built the Egyptian Pyramids instead of humans. For this to be true, we'd need the following givens:

# aliens exist [[note]]Not necessarily unlikely; the universe is a stupefyingly big place.[[/note]]
# they are intelligent [[note]]Less likely. [[UsefulNotes/FermiParadox While life may be common,]] human-level or above intelligence may not.[[/note]]
# they exist contemporaneously with humans [[note]]Physically modern humans have only been around for 200,000 years. At any time during those years humans could have gone extinct, and may yet go extinct in the future. In other words, on the timescale of billions of years, even if the universe gives birth to more than one intelligent species, they may never meet due to one going extinct before the other can arise.[[/note]]
# they develop interstellar/intergalactic travel [[note]]This is not only a question regarding faster-than-light travel, but social development. Human technological advancement has only accelerated in the past 300 years, which was by no means inevitable, and for our first ''150,000 years'' remained almost entirely stagnant. Knowledge may also be forgotten or lost (as has happened multiple times in human history when important libraries were burnt). In short, technological advancement is not a given. Even an intelligent species may [[MedievalStasis never advance beyond medieval or even stone age technology]].[[/note]]
# they know how to find us [[note]]Statistically false; SciFiWritersHaveNoSenseOfScale. Finding Earth among all the planets in the Milky Way galaxy would be like finding one particular grain of sand on a beach, and each grain of sand was miles apart.[[/note]]
# they can build pyramids [[note]]If they have interstellar travel, they probably can.[[/note]]
# they would not leave any evidence of their existence [[note]]Apart from the pyramids, obviously.[[/note]]
# they would waste time building pyramids. [[note]]Well, if they took the time out to find us in the great expanse of the universe, they could probably spare a little longer to make a few monuments.[[/note]]

The more normal theory only requires that:

# humans exist [[note]][[CaptainObvious This is factually true.]][[/note]]
# humans can build pyramids [[note]]It's admittedly difficult but not impossible.[[/note]]
# humans would waste time building pyramids. [[note]]Which certainly ''seems'' untrue, but humans have wasted time building other huge stone things, so, why not 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 likely.'' 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 Theorist}}s 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) [[MoonLandingHoax 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:

# were willing and able to expend untold billions of dollars more than they were ever officially allocated on smoke-and-mirrors attempts;
# were smart enough to fool 99% of the population (which some would contest);
# were simultaneously stupid enough not to cover their tracks;
# had the technological and film-making ability to actually fake the moon-landing footage[[note]]At first this seems to make sense. After all, surely it must be easier to ''fake'' some film of something rather than actually film the thing itself; but faking footage relies on a totally different field of technology. In fact, the technology to fake the moon landings ''[[http://www.youtube.com/watch?v=sGXTF6bs1IU did not exist]]'' in 1969, whilst the technology to actually go there and shoot some footage did.[[/note]];
# 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 [[UsefulNotes/RichardNixon simple burglary]];
# were able to pay off and swear to silence [[RedsWithRockets the Soviet Military (and its intelligence directorate, the GRU)]] [[BalanceOfPower 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 [[StateSec the KGB]]) without arousing their suspicions, or paid off and swore to silence all three factions despite [[UsefulNotes/HistoryOfTheColdWar the natural inclination of all three to seriously humiliate their mutual enemy]];
# 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 ([[TheGreatPoliticsMessUp 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 UsefulNotes/TheNewRussia as well.

After that, you'd think that the simplest explanation was to, you know, actually send people there (''Series/ThatMitchellAndWebbLook'' has a brilliant series of sketches on this idea, [[http://www.youtube.com/watch?v=P6MOnehCOUw 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 RealLife 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) [[UsefulNotes/IsaacNewton 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, UsefulNotes/AlbertEinstein--to formulate more complex theories, particularly "UsefulNotes/{{Relativity}}"[[note]]Einstein explicitly made this point himself when he said that ideas should be kept as simple as possible, ''but not simpler''[[/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 [[Series/{{House}} Dr. House]] (who actually declared Occam's Razor to be a fallacy), 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, [[http://www.telegraph.co.uk/health/healthnews/9972574/Five-year-old-girl-died-from-meningitis-after-doctor-said-hospital-was-waste-of-time.html sometimes]] with [[http://www.telegraph.co.uk/health/healthnews/9692169/Parents-win-five-figure-sum-after-hospital-failed-to-spot-babys-meningitis.html 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 ArkhamsRazor, where the most bizarre solution is most likely to be the correct one.