The Appeal to ridicule reference article from the English Wikipedia on 24-Apr-2004
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Appeal to ridicule

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The logical fallacy of appeal to ridicule presents the opponent's argument in a way that appears ridiculous and mocking it.

Examples:

  1. If Einstein is right that would mean that when I drive my car it gets shorter and heavier. That's crazy!
  2. You can't have a cat called Fido. Calling a cat Fido would just be silly!

This should be contrasted with reductio ad absurdum which is a valid logical argument form. Ridiculing the person making the argument is specifically an ad hominem attack.