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We are currently learning about fallacies and need help to clarify our understanding.

Take these 2 arguments:

  1. When walking downtown, the majority of people I asked told me that the metro was on the left; therefore, the metro is on the left.
  2. When walking downtown, the majority of people I asked told me that the metro was on the left; therefore, it is reasonable to assume that the metro is on the left.

Would both pf these arguments be considered invalid and both be an example of the appeal to popularity fallacy or is the second argument not an example of this fallacy. If the second example is not an example of this fallacy why?

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  • Do you know about the difference between validity and soundness in logic? Neither argument is valid as stated, though either one could be made valid with an additional premise, in the first case "if the majority of people I asked told me that the metro was on the left, then the metro is on the left" and in the second case "if the majority of people I asked told me that the metro was on the left, then it is reasonable to assume that the metro is on the left". But whether such an argument would be sound would depend on whether we accept the premises as true.
    – Hypnosifl
    Aug 7, 2021 at 2:29
  • The distinction is that on the one hand you’re saying that something is true because lots of people think that it is (obviously fallacious) while on the other you’re saying that something is probably true because lots of people believe that it is - this is a statistical claim and may well be correct. Either way, the location of the metro does not in any way depend on where people believe it to be, which is really the crux of the matter.
    – Frog
    Aug 7, 2021 at 2:34
  • The term "reasonable" does a lot of work in the 2nd statement. So much so that it's impossible to assert the validity of the proposition without context. Although it seems perfectly reasonable to believe the metro is on the left, it would be very unreasonable to believe Elvis is still alive only because many people told you so.
    – armand
    Aug 7, 2021 at 2:38
  • Despite the answer below, the first one is definitely the appeal to popularity fallacy; he is overthinking it
    – Al Brown
    Aug 7, 2021 at 8:33

2 Answers 2

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Well, neither are deductively valid. It is possible in case (1) that the metro could be on the right and everybody is crazy. It is possible in case (2) that everybody is playing a prank and you should have known, so that it might not even be reasonable to conclude the metro is on the left.

But neither are fallacious. They are both inductively reasonable. Argumentum ad populum is not always a fallacy; for empirically observable, non-controversial facts that the general public would be expected to know, it can be reasonable to conclude that the general public's opinion is correct. One will not always be right in this conclusion, but it's a cognitive rule that works more often than not.

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The relevant distinction is between "valid" and "cogent." Valid is a subtype of cogent. "Valid" only applies to deductive arguments; it means the truth of the premises guarantees the truth of the conclusion. "Cogent" applies to any argument; it means the truth of the premises evidentially support the truth of the conclusion (i.e. they don't necessarily prove the conclusion but they make the conclusion more probable; validity is a subtype of cogency because proving the truth of the conclusion is one way of making the truth of the conclusion more likely). Neither (1) nor (2) are valid, but (2) is cogent (in most cases; "causative" gave an example of a case in which it wouldn't be cogent). Argumentum ad populum is an informal fallacy because it only applies to informal reasoning (formal reasoning = arguments translatable into some system of symbolic logic relying only on that system's specified rules of inference) and it's only applicable under certain conditions (i.e. it's only applicable if majority opinion or the selected sample of popular opinion is irrelevant to the truth of the proposition under consideration).

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