Two twin fallacies

by Tim Harding

Seasoned skeptics may be familiar with two well-known logical fallacies:

  1. The ‘Argument from Personal Abuse’ or the ad hominem argument (playing the man instead of the ball); and
  2. The deductive form of the ‘Argument from Authority‘ or ‘Appeal to Authority’.

When you think about it, these fallacies make the same error of logic – they both draw conclusions from the character or motives of the arguer rather than the premises and form of the argument.

In informal logic, these are known as fallacies of defective induction, where it is argued that a statement is true or false because the statement is made by a person or source that is commonly regarded as authoritative or not authoritative.  The most general structure of this argument is:

   Premise 1: Source A says that statement p is true.
   Premise 2: Source A is authoritative.
   Conclusion: Therefore, statement p is true.


  Premise 1: Source B says that statement p is true.
  Premise 2: Source B is a ‘bloody idiot’.
  Conclusion: Therefore, statement p is false.

We skeptics are often skeptical of conspiracy theories, such as the so-called Moon Landings Hoax.  Conspiracy theories like these are often a special case of the ad hominem argument, for example:

   Premise 1: NASA claims to have landed men on the Moon;
   Premise 2: Governments can’t be trusted;
   Premise 3: NASA is a government agency;
   Conclusion: Therefore, NASA’s claim is false.

These arguments are fallacious because the truth or falsity of the claim is not necessarily related to the attributes or motives of the claimant, and because the premises can be true, and the conclusion false (an authoritative claim can turn out to be false).  If the premises can be true, but the conclusion can be false, then the argument is logically invalid. (A logically valid argument is one where if the premises are true, then the conclusion must be true by virtue of the argument’s logical structure).

An exception can be made for an ad hominem argument if the attack goes to the credibility of the arguer. For instance, the argument may depend on its presenter’s claim that he’s an expert. (That is, the ad hominem argument is undermining a legitimate Argument From Authority). Trial judges allow this category of refutation in appropriate cases.

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Filed under Logical fallacies

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