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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. [[SubvertedTrope 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]]''' is a logical principle 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 zebras."

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]]This is could be true; the Milky Way is a big place, and the universe is a stupefyingly big place[[/note]]
# they have some form of interstellar travel[[note]]This is certainly not impossible, but it'd be difficult to falsify; our modern science today hasn't the faintest clue how to get to other stars, and the farthest-flung manmade object--[[http://en.wikipedia.org/wiki/Voyager_1 Voyager 1]]--is not even a light-''day'' from Earth[[/note]]
# they know how to find us[[note]]Statistically false; SciFiWritersHaveNoSenseOfScale[[/note]],
# there is absolutely no other evidence to their existence other than the pyramids[[note]] some, however, would dispute this [[/note]],
# they would be able to both work stone and leave fake proof that fools entire civilizations, and
# once they got here, they'd waste time [[{{Troll}} building huge stone things in a desert]] instead of, you know, actually showing themselves.

The more normal theory only requires that:

# humans exist[[note]][[CaptainObvious This is factually true]], UsefulNotes/{{Solipsism}} notwithstanding[[/note]], and
# said humans would waste time building huge stone things in a desert[[note]]Which certainly ''seems'' untrue, but humans have wasted time building huge stone things elsewhere, so why not a desert?[[/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, which was more likely to be the case, and thus encouraging people to reduce the conditions that caused them to be exposed to stress, and to eat a bland diet, were reasonable treatments for people with stomach ulcers, until 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.

Occam's Razor is the bane of {{Conspiracy Theorist}}s everywhere for the same reason: take a look at 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:

# were willing to throw billions away for 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]]This seems logical - after all, surely it would be easier to fake some film than actually ''go to the Moon'' - but it relies on comparing two totally different fields. 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 [[RichardNixon simple burglary]], and
# were actively collaborating with the Soviets during a period of history where relations were historically edgy and were given consent by Moscow to win this symbolic victory. Alternatively, the Soviets (who were monitoring all of our launch activities and radio transmissions) declined to call shenanigans on the whole thing.

After that, you'd think that the simplest explanation was to, you know, actually send people there (''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) [[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, AlbertEinstein--to formulate more complex theories, particularly the outrageous stew we call "UsefulNotes/{{Relativity}}" which functions along completely different rules. Now, Occam's Razor would suggest that there must be some Grand Unified Theory that explains why physics work one way on an atomic level and completely differently on a larger-than-atomic level. Much of the last century of scientific research (including Einstein's) has centered around trying to come up with one. They haven't succeeded. So far, Occam's Razor is wrong, and the universe simply functions according to completely different sets of rules depending on an object's physical size, for no good reason whatsoever. Nobody likes this, but in the end, nothing says that an explanation must ''be'' simple.

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, 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.