Appeal to Ignorance - Definition & Examples

An appeal to ignorance treats the absence of evidence against a claim as though it were evidence for it. Because nobody has shown the claim to be false, it is taken to be true — or, in the mirror version, because nobody has shown it to be true, it is taken to be false. In both directions the argument draws a positive conclusion from nothing more than a gap in what is known.

Example of Appeal to Ignorance

Priya: "There is no scientific proof that this supplement does anything." Dale: "And no one has proved that it doesn't work, so I'll keep taking it." Dale treats the absence of evidence against the supplement as evidence in its favour. The studies have not come back negative; they have not been done.
"Despite years of investigation, no one has ever produced hard evidence of the conspiracy — which just shows how well it has been covered up." Here the lack of evidence is converted into evidence for the claim, which makes the position unfalsifiable: any absence counts in its favour.
"There's no proof that the new road caused the drop in trade, so it didn't." The mirror form. Nobody has established the link, but that on its own does not establish there is none — particularly if no one has looked.

Note

Alternative Names: Argumentum ad Ignorantiam, Argument from Ignorance, Appeal to the Unknown

In more detail

The appeal to ignorance, or argumentum ad ignorantiam, substitutes the limits of current knowledge for evidence. "No one has disproved it" describes the state of the discussion, not the state of the world, and a claim that has never been tested is not thereby supported.

The fallacy has two symmetrical forms. The first infers truth from the absence of refutation: since there is no proof that ghosts do not exist, they must. The second infers falsehood from the absence of confirmation: since no one has proved the treatment works, it must be useless. The second form is the easier one to miss, because it can masquerade as appropriate scepticism.

It is closely related to the Burden of Proof fallacy and the two often appear together, but they are not the same move. Burden of proof concerns who is obliged to supply evidence; the appeal to ignorance concerns what may be concluded when that evidence never arrives. Someone who says "you cannot disprove my claim, so you must accept it" has committed both: they have shifted the burden, and then treated the resulting silence as support.

Absence of evidence is not always uninformative, and this is where care is needed. Where a thorough search would reliably have turned something up, finding nothing is genuine evidence that it is not there — a drug trial large enough to detect an effect, finding none, really does count against the effect. What makes the reasoning fallacious is assuming that the search was thorough enough to license the conclusion, rather than showing it. The related Argumentum ex Silentio is the historical version of the same question, applied to what a record fails to mention.