Ad lapidem is a logical fallacy that consists in dismissing a statement as absurd without giving proof of its absurdity. The form of argument employed by such dismissals is the argumentum ad lapidem, or appeal to the stone.

Ad lapidem statements are fallacious because they fail to address the merits of the claim in dispute. Ad hominem

Ad hominem

An ad hominem , short for argumentum ad hominem, is an attempt to negate the truth of a claim by pointing out a negative characteristic or belief of the person supporting it...

arguments, which dispute the merits of a claim's advocate rather than the merits of the claim itself, are fallacious for the same reason. The same applies to proof by assertion

Proof by assertion

Proof by assertion, sometimes informally referred to as proof by repeated assertion, is a logical fallacy in which a proposition is repeatedly restated regardless of contradiction. Sometimes this may be repeated until challenges dry up, at which point it is asserted as fact due to its not being...

, where an unproved or disproved claim is asserted as true on no ground other than that of its truth having been asserted.

The name of this fallacy is attributed to Dr. Samuel Johnson

Samuel Johnson

Samuel Johnson , often referred to as Dr. Johnson, was an English author who made lasting contributions to English literature as a poet, essayist, moralist, literary critic, biographer, editor and lexicographer...

, who refuted Bishop Berkeley's immaterialist philosophy (that there are no material objects, only minds and ideas in those minds), by kicking a large stone and asserting, "I refute it thus." This action, which fails to prove the existence of the stone outside of the ideas formed by perception, fails to contradict Berkeley's argument, and has been seen as merely dismissing it.

Examples

Albert claims that Bob has embezzled from his employer.

Carl replies, "That can't be true. Bob is not an embezzler, not now or ever. He simply wouldn't do such a thing. It's nonsense, a ridiculous claim on its face."

Because it denies Albert's claim, but gives no reasoning or evidence proving it untrue, Carl's reply is an argumentum ad lapidem in form. In making it, Carl has committed the ad lapidem fallacy, and Albert would be reasonable to discount it.

Another example:

• Speaker A: Infectious diseases are caused by microbes.
• Speaker B: What a ridiculous idea!
• Speaker A: How so?
• Speaker B: It's obviously ridiculous.

Speaker B gives no evidence or reasoning, and when pressed, claims that Speaker A's statement is inherently absurd, thus applying the fallacy.