fallacy

Argument from ignorance

Also called: Appeal to ignorance, Ad ignorantiam

An argument from ignorance claims something is true because it has not been proven false, or false because it has not been proven true.

How it works

A lack of evidence is treated as evidence for the opposite. Absence of proof can matter when a thorough search would have found something, but usually it only shows that we do not know yet.

Examples

"No one has proven the new chemical is harmful, so it's safe."

Safety needs positive evidence from testing, not just an absence of studies.

"You can't prove the meeting didn't happen, so it did."

The burden lies with the person claiming the meeting took place.

How to spot it

"Nobody has disproved..." or "there's no evidence that it isn't...".
The burden of proof is placed on the sceptic.
Unknowns are turned into conclusions.

How to respond

Ask who made the claim and what evidence supports it.
Distinguish "no evidence of" from "evidence of no".
Accept "we don't know" as a legitimate answer.