Identifying and labeling logical fallacies when they are used as argument strategies is useful – It weakens the rhetorical effect of the labeled argument, possibly even disqualifying it as viable support for a position.
But common fallacies can be used not only to legitimately point out truly inductively weak, logically invalid, or otherwise unpersuasive arguments, but may be overextended as well – they may be misapplied to label sound, cogent, and persuasive arguments as fallacies if and when this is not the case.
The appeal to authority:
This is often used to dismiss a position as merely an argument from authority, if and when it is actually an argument by authority – that the claimed credentials and qualifications of the authority are both true and relevant to the matter discussed, and the authority appealed to has a genuine basis for making their statements.
“Oh, that’s just something that those Establishment archaeologists say to hide the Truth about the Mayan pyramids!”
This can often be used to deny an actual causative correlation that has been shown real, claiming even then that “correlation is not causation,” and invoking a more complex causative relation than needed when the evidence may well point to the simpler relation that A causes B.
“Actually, the warming of the climate is not caused by human industrial pollution, it’s really just a natural cycle that correlates instead with the warming of Mars – and cow farts!”
This can be used to argue that the critic of an idea is attacking the proponent of an idea rather than the idea itself – note that an insult, by itself, is not an ad hominem. – it only becomes that when the insult is used as a reason that the one insulted is wrong without substantially addressing the argument itself. An ad hominem is not always a fallacy and can also be used in a legitimate way, as in pointing out a real and relevant conflict of interest or bias in the subject.
“You just say that because you’ve closed your mind to the very possibility of the unconventional.”
“I don’t trust anything you say…you’re in the pay of those well-funded liberal environmental lobbyists.”
“Scientists are arrogant for claiming they know anything.”
Reductio ad absurdum:
Like some other fallacies, this may be used as part of an inductively strong argument or logically valid one and is often used in formal logical proofs. It becomes a fallacy, a false reductio ad absurdum, and a straw man(see below) when used to argue the silliness of a position without using the actual, original line of reasoning in the argument.
This one is easy to commit, and easy to overextend when applied to a legitimate critique of one’s position using the premises and logic actually involved in the original argument. To avoid this, it is necessary to do whatever is required to understand an opponent’s argument and interpret it as charitably as possible, without over-generously ignoring non sequiturs and inconsistencies, or being too proud to ask for clarifications.
To overplay this fallacy and falsely accuse your opponent of a straw man is to commit one yourself through misunderstanding the counterargument given.
An example of both a false reductio ad absurdum and a common straw man:
“If you don’t believe in psychic powers, then you must also not believe in dark energy and dark matter, so 90% of the universe must not exist, because you haven’t seen those either!”
This form of reasoning is not itself innately fallacious, and can be a perfectly good logical strategy for constructing hypotheses for testing. The fallacy comes when it is used to dismiss fair criticism of an idea or used in an ad hoc manner to patch together a set of hypotheses in an overly limited fashion and render them untestable – neither falsifiable nor meaningfully verifiable. It is also over-employed when used to criticize a valid or strong argument as being ad hoc, when in fact the argument’s premises and assumptions are supported through prior evidence, arguments, or observations and the reasoning is not overly baroque in structure.
“Psi is real, and has been successfully replicated, but skeptical readers of journals these studies are published in use an unconscious, retroactive, and unobservable psychokinesis that reaches through time and causes the successful replications to fail.”
“The big bang model of cosmology can’t possibly be viable…it’s got too many patches like fairy-tale dark matter, undetectable dark energy, and imaginary inflation propping it up from falsification by protecting it from the data.”
The Fallacist’s fallacy…
…which is to argue that because an argument is invalid or weak that the argument’s conclusion is therefore false.
This shows a misunderstanding of the relationship between the truth value of the conclusion and the nature of validity or strength.
Deductive validity means that if the premises are true then the conclusion follows automatically – There is no valid logical argument in which the premises can be true and the conclusion false, because the chain of reasoning follows with certainty, but it is possible for an argument to be fallacious and still have a true conclusion – it just doesn’t follow from the reasoning, requiring a better argument for one’s position.
A conclusion can be false, even with true premises, and an argument therefore not follow, but not following from the premises does not imply the falsehood of the conclusion, only that the argument itself is not cogent, is unpersuasive, and cannot be used to support that position.
Arguments BTW, cannot themselves be true or false, only the individual statements making them up.
(Updated as of 2017.06.06)