Appeal to Authority

I hadn't planned to jump into irrelevant appeals just yet, but it comes up so frequently I can't resist.

Appeal to Authority
Other name(s): Argument From Authority, Appeal to Inappropriate Authority, Ipse Dixit ("He, himself, said it"), Argumentum ad Verecundiam ("argument to respect")
Type: Informal
Category: Red Herring (Irrelevance)
Subcategory: Genetic (Irrelevant History)

Definition
Arguing from the idea that a person considered an authority affirms a proposition, and therefore the proposition is true. Authority is used to falsely establish the final word.

Usual Form
Statement is made that because authority A states proposition P, P is necessarily true.

Explanation
Obviously, not all appeal to authority is fallacious. That is why we have authorities and experts to begin with: no one can know everything. But since even the top authority on a subject can affirm an untrue proposition, knowingly or unknowingly, appeals to authority are all deductively (formally) fallacious. That is, they lack airtight logic simply by their form.

However, Appeal to Authority is listed among informal (inductive) fallacies because it has to do with the content of the argument. Not all appeals to authority are examples of logical fallacy. It applies when a referenced authority is either not an authority in the relevant area, not an authority at all, or has untrustworthy positions due to bias. A position well apart from those of other similar authorities can also invoke this fallacy, since it is possible to find some "expert" somewhere who will agree with anything. This also applies when an expert view is either unnecessary or impossible.

Dr. Gary Curtis, whose excellent (if often challenging) site on logical fallacies is among those I'm using to assemble knowledge on the subject, has a good analysis on this subject, which he sums up in a checklist to see if authority is to be relied upon:

• Is this a matter which I can decide without appeal to expert opinion? If the answer is "yes", then do so. If "no", go to the next question:
• Is this a matter upon which expert opinion is available? If not, then your opinion will be as good as anyone else's. If so, proceed to the next question:
• Is the authority an expert on the matter? If not, then why listen? If so, go on:
• Is the authority biased towards one side? If so, the authority may be untrustworthy. At the very least, before accepting the authority's word seek a second, unbiased opinion. That is, go to the last question:
• Is the authority's opinion representative of expert opinion? If not, then find out what the expert consensus is and rely on that. If so, then you may rationally rely upon the authority's opinion. If an argument to authority cannot pass these five tests, then it commits the fallacy of appeal to misleading authority.

There are many subspecies of Appeal to Authority, and odds are I'll try to cover them eventually here.

Examples
"It's in print, so it must be true!"
My brother actually heard this from one of his Jr. High School teachers.

"The Pope says that priests can turn bread and wine into Jesus’ actual body and blood. He says it's a physical change which happens to it, and I believe him."
The Pope may believe what he says, and odds are the Pope is not a liar, but His Holiness is not an authority on the fact that the bread and wine are actually transformed. An authority would be someone who has objectively observed whether the change took place.

Many, many examples of this can be found all over social media sites, with, for example, pseudo-scientists being quoted to support all manner of nonsense.