Argument from Ignorance

Argument from ignorance (argumentum ad ignorantiam or appeal to ignorance) is an informal fallacy that claims a proposition is true because it has not been proven false, or false because it has not been proven true. The typical forms are: "P has not been proven false, therefore P is true" and its converse. The term likely dates to John Locke and is often used to shift the burden of proof. While usually fallacious, there are limited contexts where lack of contrary evidence reasonably supports a conclusion - for example, the legal presumption of innocence where not proving guilt means the defendant should be treated as not guilty.

Concrete illustrations include the contrived "moon made of spare ribs" example, a common-job-callback inference ("they never called, so I didn't get the job") and public debates like prewar claims about weapons where absence of evidence was invoked. Critics emphasize that absence of evidence is not evidence of absence, a point Carl Sagan and others stress. Related concepts clarify when such reasoning can be valid: contraposition/transposition is a legitimate logical move; a scientific null result can sometimes constitute evidence of absence; and the related "argument from self-knowing" (if P were true I would know it, I do not, therefore not-P) depends entirely on the truth of its premises and is often unsound.