Occam's Razor

The simplest shave you'll ever get, folks.

"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."

— No Edge 2: The Curvature of the Universe

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. Occam's Razor is often paraphrased, "The simplest explanation is the best one."

Unfortunately, this can lead to misunderstandings, at least so far as what is meant by "simplest." The actual principle is, "Entities must not be multiplied beyond necessity" (or "Entia non sunt multiplicanda praeter necessitatem"). The best explanation is the one which makes the fewest assumptions and explains all the data. This means a highly complex answer may actually be the better one if an alternative, deceptively simpler answer fails on one or both counts (it may be easier to understand but requires unproven assumptions, or it fails to explain all the data). Also important to remember is that an explanation that previously accounted for all the data may fail to do so in the future when new data appears, and so requires refinement or even replacement.

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, all else being equal, believing the theory with the fewest underlying premises. When one really starts looking under stones, one can find an apparently simple explanation is actually underpinned by a whole host of assumptions that at first glance had gone unnoticed.

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 This is certainly plausible. The universe is a stupefyingly big place; it is not unlikely that life exists somewhere besides Earth.
2. They are intelligent^note Less plausible. While life may be common, human-level or above intelligence may not be, especially depending on how one might define the word "intelligence".
3. 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.
4. They're more advanced than us^note Conversely to the "late to the party" theory, there's a new theory that humans might actually be one of the first intelligent species in the universe, rather than newcomers in a universal civilization
5. 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 never advance beyond medieval or even stone-age technology. Also, this and the above points involve the assumption that we're dealing with the Rubber-Forehead Aliens common to sci-fi; real-life aliens could be of a form that we humans could never fathom and may never be able to develop or use technology as we do, however intelligent they may be.
6. They would find Earth^note Sci-Fi Writers Have No Sense of Scale. Space is big. Really, really big. However big you're imagining space is, it's even bigger than that. Even if faster-than-light travel is possible and the hypothetical aliens have the time, resources and inclination to develop it and originate in the same galaxy as us and live at the same time we do, it would still take an unfathomable amount of time to explore the Milky Way even to just the cursory extent required to identify every planet and figure out which ones might be interesting enough for a closer look. Since there are at least 100 billion planets in our galaxy, it's very unlikely that Earth will be among the first planets they visit.
7. They can build pyramids^note If they have interstellar travel, they probably can.
8. They would build 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.
9. They would not leave any obvious evidence of their existence^note It's unlikely that they'd intentionally avoid being learned about by lifeforms after them, especially after building those incredibly conspicuous pyramids. If they cared about going unnoticed, they wouldn't be building stuff. And if they did come all the way to Earth and were inclined to build large monuments, why would they design said monuments to look as if they were built using simple stone and bronze tools?

The more normal theory only requires that:

1. Humans exist^note This is factually true.
2. Humans can build pyramids^note Conspiracy theorists might dispute this fact, but even if it was difficult for ancient humans to build pyramids, it could not have been literally impossible. Humans have been observed, worldwide, to stack stones in neat orderly triangular piles called "cairns".
3. Humans would build pyramids^note Humans have wasted time building other huge stone things, so why not pyramids?

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.) In other words, when you hear hoofbeats, think horses, not zebras.

Occam's Razor is the bane of Conspiracy Theorists everywhere since conspiracy theories usually rest on a lot of shaky assumptions. For example, take the Apollo Moon Landings, which many believe was a hoax. At its base, the conspiracy claims that the United States was not technologically able to send a man to the Moon, and so had to fake it. However, when you really get into what would be necessary for such a conspiracy to be accomplished, you realize it'd actually be simpler and easier to just send people to the Moon. (That Mitchell and Webb Look has a brilliant series of sketches on this idea, including the moon landing).

As mentioned above, the Razor requires that all the data is accounted for. Newtonian physics are simpler than modern theories and were sufficient to take a man to the Moon, but Sir Isaac simply could not explain all the new data collected after him — most of which was completely unknown when Principia Mathematica was published. This required another smart man — namely, Albert Einstein — to formulate more complex theories, particularly "Relativity." Einstein explicitly made this point himself when he said that ideas should be kept as simple as possible, and not simpler.^note Meaning, keep to the simplest explanation that actually works, but do not oversimplify to the point that anything actually important gets removed. It should be noted that Newton's theories weren't simply thrown in the trash. While Einstein's theories are concerned mainly with objects traveling at relativistic speeds and objects with extreme mass (such as black holes), Newton's theories are still used for macroscopic bodies so long as the mass and speed are not too big.

Another very common mistake is to summon up the Razor in a debate in order to add weight to a particular argument. Occam's Razor is a mental guide — nothing more — and cannot by itself be used to validate or invalidate any particular theory. This usage is entirely fallacious, as the Razor does nothing more than recommend searching for hypotheses that make the fewest new assumptions and explain all the data. It is not a magical baton that points to the right answer. Applying it in this way inevitably leads to a frankly embarrassing race to the bottom over whose theory is "simplest." Any hypothesis for a given question still has to be tested, no matter how "simple" it might be.

The inverse of this is Arkham's Razor, where the most bizarre solution is most likely to be the correct one. See also Hanlon's Razor, where events attributed to malicious intent are more easily explained as the result of someone's stupidity.

No examples, please. This only defines the term.

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