Critical Thinking: What is the Fallacy of Appeal to Ignorance?

Part of critical thinking is learning to recognize-and avoid falling prey to-common logical fallacies. One of these fallacies is called appeal to ignorance.

And no, appeal to ignorance isn't an accusation that a person putting forth a certain argument is "ignorant." It's not just a bit of name calling.

An appeal to ignorance is an argument form whereby the only grounds offered for accepting a certain claim are that there are insufficient reasons to disbelieve it.

Let's look at some examples to make this clearer:

* Premise: Delaney doesn't have an alibi that proves he wasn't at the scene of the crime.

* Conclusion: Delaney was at the scene of the crime.

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* Premise: There have been no sightings or other evidence to indicate there is a planet between Earth and Mars.

* Conclusion: There is no planet between Earth and Mars.

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* Premise: No one has ever proven that reincarnation is false.

* Conclusion: Reincarnation is true.

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Not all appeals to ignorance are fallacious, or equally fallacious. You may have heard it said that "Absence of evidence is not evidence of absence," but that's false. It most certainly is evidence, it's just that sometimes it's very slight, very weak evidence, sometimes it's evidence of middling strength that's still well short of conclusive, and sometimes it's very strong evidence.

And that's what you need to look at in deciding how much weight to give an appeal to ignorance. You need to ask yourself if it's the kind of situation where hypothetically if the conclusion were false we'd have evidence to show that (and hence since we don't have such evidence, the conclusion is likely true).

For example, let's say you want to know if you may have left your cell phone at Cecil's or at Millicent's. Cecil glances around the room for five seconds from where he's sitting, doesn't see anything, and tells you you didn't leave it at his place. Millicent spends two hours going through every room in her house, looking through every drawer, pulling out furniture to look behind it, etc., doesn't see it, and tells you you didn't leave it at her place.

In effect, they're both basing their conclusion on the same premise:

* Premise: I haven't seen your cell phone at my house.

* Conclusion: Your cell phone isn't at my house.

So in both cases their reasoning takes the form of an appeal to ignorance, where the support offered for the conclusion is an absence of evidence for the alternative.

But note that their arguments differ greatly in strength, and note why this is so. Cecil is much more guilty of a fallacy here, because he has only the tiniest of grounds to be able to say "If your phone were here, I'd have seen it." Millicent, however, has very good grounds for saying precisely that. So the fact that she hasn't seen your cell phone matters a lot more than that Cecil hasn't. Neither can "prove" their conclusion, but Millicent's conclusion is strongly supported, even though her only premise is a negative one-that she has no visual evidence of your cell phone's presence in her house.

So if we think about our three earlier examples in these terms, we can get a sense of how strong or weak they are.

* Premise: Delaney doesn't have an alibi that proves he wasn't at the scene of the crime.

* Conclusion: Delaney was at the scene of the crime.

Depending on the circumstances, this is probably quite weak and constitutes a fallacy. The mere fact that someone can't prove they weren't somewhere doesn't establish that they were there. (That is, unless we imagine some really unlikely set of circumstances where the only place Delaney could have been other than the crime scene is somewhere that it would be easy for him to establish that that's where he was. Like if Delaney is claiming he temporarily left the island where the crime occurred, but the only way on and off is a boat service to the mainland that meticulously records the name of every passenger. In that case, if there is no evidence-i.e., no listing of his name in the boat records-indicating that he was on the mainland the night of the crime, that absence of evidence does make it likely he was on the island.)

* Premise: There have been no sightings or other evidence to indicate there is a planet between Earth and Mars.

* Conclusion: There is no planet between Earth and Mars.

This appeal to ignorance is actually quite a strong argument. There is little or no fallacy here, because it's extraordinarily unlikely something the size of a planet could exist so close to us and somehow there be no evidence of it. "Absence of evidence" certainly is "evidence of absence" in this case.

* Premise: No one has ever proven that reincarnation is false.

* Conclusion: Reincarnation is true.

This is an example of a fallacious appeal to ignorance. One really needs to make a better case for reincarnation than simply "Well, nobody's ever proven it can't happen!" Assuming it were false, what conclusive evidence would exist for this? I can't imagine. So the absence of such "proof" tells us little or nothing.

(An argument that reincarnation is false that relied solely on the claim that no one has ever proven it's true, would be a poor argument for the same reason. Indeed one of the ways we know "appeal to ignorance" is a fallacy is that if it were not, you could take any issue about which there's little or no evidence-let's say something like whether there are an odd or even number of grains of sand in the Mojave Desert-and equally well "prove" both sides.)

In conclusion, normally, but not always, it takes considerable positive evidence to justify believing something. An appeal to ignorance is an argument that cites only something negative-the absence of evidence for the other side-and so is normally, but not always, a fallacy.