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Ad Hoc

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Ad hoc is a fallacious debating tactic (also called a "just so story" or an "ad hoc rescue") in which an explanation of why a particular thing may be is substituted for an argument as to why it is; since it is therefore not an argument, it is not technically a fallacy, but is usually listed as one because it is a substitution for a valid argument. It is similar in form to Moving the Goalposts, but protects the argument by adding additional speculative terms rather than changing the meaning of existing ones.


Users of ad hoc claims generally believe the excuses and rationalisations serve to shore up the original hypothesis, but in fact each additional speculative term weakens it. This is both due to the speculations being based simply on the faith that there might be an explanation, and because each additional term makes the hypothesis weaker according to the principle of Occam's Razor.

"Possibly," "probably," "maybe," "might" and "could" are all good markers of ad hoc claims.

It's a very common sight in justifying edits aimed at any supposedly negative trope, particularly if that edit calls upon things that might have happened to cause the item described. For example:

"There is a glaring Plot Hole in this movie, since it is said that magic cannot work in the Dark Zone, yet later on Alice resurrects Bob while fighting the Phantom Lord."
"It's possible that Alice learned a form of magic that does work in the Dark Zone, and that's what she used to save Bob."

Here the second poster is not presenting evidence: rather, they are explaining what the evidence they do not have ought to look like.

Looks like this fallacy, but isn't:

  • When an opponent makes the argument "A, therefore B" there are two ways to rebut it: either show A is false, or that A does not necessarily lead to B. The latter rebuttal can be speculative without being ad hoc. Explanation: there are two types of causes: necessary and sufficient. A necessary cause must exist for the effect to happen, but may not be enough by itself to cause the effect. A sufficient cause can create the effect all by itself, but may not have to exist for the effect to happen. Oxygen is a necessary cause for fire; no oxygen, no fire, regardless of anything else that might be going on. Oxygen is not a sufficient cause for fire; lots of other things have to happen as well as there being oxygen around. Decapitation is a sufficient cause of death; decapitation will kill you without anything else being required. It is not a necessary cause of death; there are all kinds of causes of death which leave the head attached.
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  • A simple way to see this is to remember to SiN, where Sufficient Implies Necessary. In other words, review the sufficient and necessary statements as an If-then statement, then the sufficient should naturally come first. If you are decapitated, then you will die. (Death as necessary for decapitation, decapitation is sufficient for death.) If you eat a thousand donuts then you will gain weight. (A thousand donuts will surely make you heavier - in other words, is sufficient, but it is hardly necessary for making you heavier.) If you browse TV Tropes, then you will waste hours on the internet. (Directing someone to TV Tropes is a sufficient for them wasting hours upon hours online, but there are addictive sites out there.) As a tool, it is useful for conceptualizing the difference, but do not argue logically based on it.

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