John H. Watson and Sherlock Holmes from an 1893 illustration ofThe Greek Interpreter

Cogito ergo sum

Key articles

Logical fallacy Syllogism Argument

General logic

Gödel's incompleteness theorems Enthymeme Nirvana fallacy Invincible authority Appeal to the minority Regression to the mean

Bad logic

Quote mining Sunk cost How do you know? Were you there? Appeal to faith On the spot fallacy Viés de seleção v -t -e

AHolmesian fallacy (alsoSherlock Holmes fallacy orprocess of elimination fallacy) is alogical fallacy that occurs when some explanation is believed to be true on the basis that alternate explanations are impossible, yet not all alternate explanations have been ruled out.

The fallacy is anappeal to omniscience and aninformal fallacy.

In popular culture[edit]

The TV seriesHouse reprises the Holmesian structure but in a modern medical setting, replacing detectives with diagnosticians. The diagnosticians try to solve unlikely medical cases by whittling downjargon-laden possible diagnoses (the differential diagnosis) into an improbable last-minute treatment.

Alternate names[edit]

• far-fetched hypothesis
• arcane explanation

Explanation[edit]

The Holmes Method is illustrated by the quote from "The Adventure of the Blanched Soldier",^{[note 1]} a 1926 Sherlock Holmes short story:

"When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth." —Sherlock Holmes

To apply this method, one must find explanations and eliminate them one by one. However, to draw a logical conclusion from this is fallacious, because both steps require omniscience:

1. Findevery possible explanation.
2. Correctly disproveevery possible explanation, except the true and undisprovable one.

As should be obvious, this is incredibly difficult, requiresall knowledge of the situation, and may lead one to ridiculously improbable explanations.

In essence, a major flaw in this line of reasoning is that there may be explanations that you simply have not thought of.

Far-fetched hypothesis[edit]

It is typically offered byghost enthusiasts as an example of why one must not discount belief in thespirit world.^[1][2][3]Star Trek's Spock even uttered a version of it inStar Trek VI — The Undiscovered Country.^[4] It's no surprise thatparanormal claims of all kinds have adopted this fallacy. For example,crop circles:

Look at thatmutilated cow in the field, and see that flattened grass. Since the farmer is away and couldn't have done the deed,aliens must have landed in aflying saucer and savaged the cow to learn more about the beings on our planet.

Naturally, these are not the only explanations. Angry neighbors, rival farmers, pranksters, predators (not theSchwarzenegger movie kind), undetonated ordnance from a pastwar (if applicable), and many other perfectly mundane probabilities exist that must be eliminated. Even assuming they are, there are myriads of othersupernatural orparanormal phenomena that could potentially explain the death of the cow and the flattening of the grass that would similarly need to be falsified before the conclusion of "UFOs" or "aliens" can be accepted under this logic (For example, perhaps it was attacked by anaviancryptid. Perhaps it was struck down by [Insert Deity Here]. Perhaps it was possessed by ghosts ordemons, causing the cow to burst as they departed its body. Perhaps it is evidence of an airborne biological weapons test byMonsanto, designed to kill all cattle and force America to eatGMO-fed meat. And so on.Ad infinitum.).

According toAristotle inDe Anima, the philosopher Thales of Miletus once said (paraphrased):

The lodestone has asoul because it moves iron. This proves that all things are full ofgods.^[5]

Thales failed to falsifyall alternate explanations because he didn't know thatmagnetism existed. Similarly, we may be disproven by future discoveries.

Lord Kelvin is almost infamous for these. Because he had no knowledge of nuclear fusion, isotopic decay, or nuclear fission, there was in his mind no way to reconcile the evidence that had been found bybiologists andgeologists that theEarth is at least hundreds of millions of years old with the seeming impossibility of theSun shining almost as bright as it does today for that period. As such, he believed the Sun was at most a few million years old and that either the biologists and geologists were wrong or the Earth far predated the Sun.

Unfalsifiable explanations[edit]

Most often, the Holmes method leads people to explanations that areeasy to find but difficult or impossible to falsify, such as agod of the gaps or othersupernatural entities with unknown properties. Often, people ignore the flaws in their preferred explanations.

Arthur Conan Doyle, who wrote the Sherlock Holmes stories, himself ended up believing inspiritualism andfairies by "using" the Holmes method.

Legitimate use[edit]

It is possible to grant provisional (scientific, not logical) assent to an explanation if it explains something better (ormore simply) than all known alternatives. This isn't anabsolute truth, merely the mostprobable one.

An alternate formulation, that is closer to what Holmes actually practiced, rather than preached, is that "when you have eliminated the common possibilities, one of what remains must be truth, no matter how improbable what remains may be".

Dirk Gently, in Douglas Adams'The Long Dark Tea-Time of the Soul, rejects the Holmes principle on the basis that "the impossible often has a kind of integrity to it which the merely improbable lacks." Often, impossible ideas merely suppose that there is something we don't know about, "and God knows there are enough of those". This ties in to the omniscience idea, that you should not assume that all possible explanations have been accounted for just because you personally can't think of any others.^[6]

See also[edit]

• Argument from incredulity
• Appeal to omniscience
• Slothful induction
• Explanatory Filter

External links[edit]

• Beware The Logic of Sherlock Holmes, Skeptical Science
• A Sherlock Holmes Logical Fallacy, Steven Novella
• The Holmesian Fallacy, Bob Frankston
• A Fallacy of Sherlock Holmes, Philosophy 12
• Arcane Explanation, Bruce Thompson
• Far-Fetched Hypothesis, Internet Encyclopedia of Philosophy
• Skeptical Fallacies: The Holmes Retroduction, Topher Cooper

Notes[edit]

References[edit]

• Logic
• Fallacies
• Arguments from ignorance

• Pages using DynamicPageList parser function