Appeal to Ignorance

In plain terms

An appeal to ignorance argues that something is true because it hasn't been proven false, or false because it hasn't been proven true. The gap in our knowledge gets treated as if it were knowledge itself. "No one has shown it's harmful, so it's safe." "You can't prove it doesn't exist, so it does."

Not knowing something is just not knowing. It isn't a quiet vote for either answer.

Why it's fallacious

Evidence and the absence of evidence are different things. A claim is supported by reasons to believe it, not by the lack of reasons to reject it. When you treat "unrefuted" as "confirmed," you let any claim that happens to be hard to disprove stand as if it were established, which is most claims about the unobserved, the future, or the hidden.

There's a narrow, legitimate cousin of this move (covered below), but the fallacy proper draws a positive conclusion from pure absence.

Canonical example

"There's no proof that the supplement causes any harm, so it's perfectly safe to take long-term."

The absence of proof of harm is not proof of safety. It may mean the supplement is safe. It may also mean no one has studied it properly, or the studies were too short, or the harm is real but undetected so far. "We haven't found a problem" and "there is no problem" are different statements, and the argument quietly swaps the first for the second.

Counter-example (not a fallacy)

"We ran a thorough search where the effect would have shown up if it existed, across a large sample, and found nothing. That absence is itself evidence the effect is small or absent."

This isn't an appeal to ignorance. When a careful investigation would be expected to find something and doesn't, the absence becomes informative. This is how drug safety monitoring and many scientific null results work. The difference is that the search was real and competent. The fallacy draws its conclusion from the mere fact that no one has looked, or from the impossibility of ever looking.

The line: is the absence of evidence the result of a genuine search that would have detected the thing, or just a gap nobody has filled?

How to fix it

If you've been linked here, check whether you're resting on actual evidence or on the other side's failure to disprove you. The fix is to supply positive support: a study, a mechanism, an observation. If the honest situation is "we don't know yet," then say that, and hold the claim as open rather than settled. "Not disproven" is a starting point for inquiry, not a conclusion.

If you're on the receiving end, separate the two states out loud: "Nobody has shown it's false, true. But nobody has shown it's true either. So the honest position is that we don't know, not that your side wins by default."