Ignorantiam

Ignorantiam, more commonly known in logical studies as the appeal to ignorance or argumentum ad ignorantiam, is a fallacy in which a proposition is asserted as true or false based on a lack of evidence to the contrary. In its standard form, the claim shifts the burden of proof: because there is no proof against a statement, the statement is declared true, or conversely, because there is no proof for it, the statement is declared false.

Two common variants are: asserting a claim is true because no evidence has proven it false, and asserting a claim is false because no evidence has proven it true. Both rely on gaps in knowledge rather than on positive support for the conclusion. For example: “No one has proven that aliens exist, therefore they do not exist,” or “No evidence shows this diet is effective, therefore it is ineffective.” Such inferences ignore the distinction between lack of evidence and evidence of absence.

The fallacy is problematic because absence of evidence is not, by itself, evidence of absence. Judgments should ideally be based on positive, credible evidence and, when evidence is lacking, on cautious, probabilistic reasoning rather than definitive conclusions. The burden of proof typically rests on the person making a claim, not on others to disprove it.

Ignorantiam appears in scientific discussions, public discourse, and pseudoscientific claims alike. Recognizing it helps distinguish undecidable cases or genuinely unsupported assertions from those backed by demonstrable evidence. Related concepts include the burden of proof, argument from ignorance, and the broader category of epistemic fallacies.