The last fallacy of relevance builds a conclusion out of what we don't know.
Key idea
An appeal to ignorance argues that a claim is true because it has not been proved false, or false because it has not been proved true.
Why it works
Not having evidence is not the same as having evidence of the opposite. No one has proved there are no ghosts in the old theater, but no one has proved there are, either. Either conclusion would be built on nothing.
Worked example: Unfair
"You can't prove I ate the last cookie, so I didn't eat it."
- The premise reports a lack of proof.
- The conclusion claims a fact.
- Nothing connects the two.
Answer: Appeal to ignorance.
Worked example: A careful search
"We checked every room, closet and shelf in the house and found no sign of the cat. So the cat is probably not in the house."
- This is not a mere lack of proof: there was a search that would very likely have found the cat.
- A failed careful search is evidence.
Answer: Fair reasoning.
Courts treat a person as innocent unless the evidence shows guilt. That is a rule about who must prove what, not a claim that a lack of proof shows innocence is a fact.
Common mistake
Treating every "we found nothing" as this fallacy. A careful search that should have found something counts as evidence ("we checked every room; the cat is not inside"). The fallacy treats a mere lack of proof as proof.