An appeal to authority becomes fallacious when a proposition is not one of relevant expert judgment. If it is an ordinary sensory observation, then the witness' expertise is irrelevant. If it is an attempt to characterize, identify, or explain an observation outside the normal ken, expertise is relevant. If the proposition is a deductive line of reasoning, then expertise is irrelevant. If it is an inductive leap outside the normal ken, then expertise is relevant.
A non-expert may certainly make a lay inductive argument, but the inductive leap belongs to the hearer, not the speaker.
"Tom's car wasn't in the garage last night and he wasn't answering his phone. I think he's having an affair."
"He could be, or perhaps he was working late and didn't want to be disturbed."
In contrast with
"I have a fever and a stuffy nose. I must have the flu."
"No, I'm a qualified doctor and the results of my medical examination are consistent more with a sinus infection."
which is an expert induction. The world doesn't stop us from drawing partially informed or inexpert judgments based on our observations. But we cannot use them as evidence in formal argumentation.
"Noted physicist Stephen Hawking told me I have the flu, not strep throat."
is fallacious because the cited expert has no
relevant expertise.
"Noted physicist Stephen Hawking once stated that if some Texans have loud voices and Tom is a Texan, then Tom must have a loud voice."
is also a fallacy because it expresses a deductive fallacy. The expertise or eminence of Hawking is irrelevant to the
inherent invalidity of the deduction.
"Noted physicist Stephen Hawking once stated that if some stars are destined to become black holes, then our sun will inevitably become one."
This is the same fallacy in different form, because it is improper deduction. However, because the nature of the statement relates to Hawking's designated field of expertise, it is not easy to notice this type of fallacy or to get a proper rebuttal to stick. A better phrased statement would be
"Noted physicist Stephen Hawking once stated he believed our sun would eventually become a black hole."
which is
not a fallacious appeal to authority, although it does raise the other important problem with expertise -- the notion that experts may differ. Earth's sun is not nearly massive enough to become a black hole, so while this fits the pattern of proper appeal to authority, the conclusion could still be contested by other experts.
"The police officer said he heard a loud noise."
is a non-authority statement. Here the officer simply acts within the ken of the normal human being, and reports a sensory observation.
"The police officer heard a gunshot."
is not a fallacious appeal to authority because a police officer could possibly be expected to distinguish a gunshot from other loud reports -- although his expert judgment would not be above challenge.
"The police officer heard a blimp hitting a building."
is not fallacious
per se but is largely non-credible because the expertise and experience of the officer would not be sufficient to judge the cause of the noise simply from hearing.
"The police officer saw what he believed to be an alien spacecraft."
"How did he know it was?"
"Because it didn't look like anything he'd seen before, and police officers are trained to be careful observers."
fails logically as well as empirically.
