Argumentum ad Ignorantiam (Latin)
Refers to a fallacy of logic in argument, specifically one of the Fallacies of Relevance. The debater using ad Ignorantiam uses an argument from ignorance.
"I've never heard of anyone flying one of these airplanes, so they don't exist!"
Compare ad hominem, argumentum ad baculum, argumentum ad misericordiam, argumentum ad populum, argumentum ad verecundiam.
Argumentum ad ignorantiam means "argument from ignorance". This logical fallacy occurs when it's argued that something must be true, simply because it hasn't been proved false. Or, equivalently, when it is argued that something must be false because it hasn't been proved true.

(Note that this isn't the same as assuming something is false until it has been proved true. In law, for example, you're generally assumed innocent until proven guilty.)
A logical fallacy where something is assumed to be true because it hasn't been proven false, or vice versa. This is used very often, and defended by insisting that the counterargument must prove the premise is false rather than prove that the premise may be false.

An example: "No studies have proven that cigarettes cause cancer, so cigarettes don't cause cancer." (this may also be a case of False Premise)

To prove Argument from Ignorance, argue that the premise in question may be false/true even if not proven.

Note that this shouldn't be confused with Proof by Contradiction, where something is shown to be true/false by proving that assuming the opposite will lead to contradiction.

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