"After we came out of the church, we stood talking for some time together of Bishop Berkeley's ingenious sophistry to prove the nonexistence of matter, and that every thing in the universe is merely ideal. I observed, that though we are satisfied his doctrine is not true, it is impossible to refute it. I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it -- 'I refute it thus.'"

The Life of Samuel Johnson, by James Boswell

Argumentum ad lapidem is the clever Latin name for what is sometimes called Proof by Assertion. It is a logical fallacy in which a person claims that their position is so obvious that it does not need any proof -- simply asserting it to be the case is sufficient to end the argument.

Argumentum ad lapidem is Latin for 'argument to the stone', and refers specifically to Samuel Johnson's stone-kicking outburst. If you think that this is an absolutely appropriate argument you might refer to it as a Moorean fact, a fact so absolutely obvious that no debate is needed. However, philosophy is based on questioning everything, so even the term 'Moorean fact' is used tongue-in-cheek.

Some people will say that argumentum ad lapidem is different from proof by assertion, as the proof by assertion refers to an argument where one intends to win by repeating themselves over and over, perhaps at volume, while argumentum ad lapidem merely involves a haughty scoff and an unwillingness to continue the discussion. It is important to note that argumentum ad lapidem also has the dubious benefit of being used by the sort of people who refer to fallacies by their Latin names and make oblique references to 18th century philosophers.

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