10.5: Appeal to Ignorance

Every Halloween night, in the comic strip “Peanuts,” Linus van Pelt makes his yearly pilgrimage to a local pumpkin patch to await the Great Pumpkin’s arrival. Many of his friends are skeptical (although Sally Brown usually accompanies him), but Linus remains convinced.

Now suppose someone offered you $50 to prove, right here on the spot, that the Great Pumpkin does not exist. Could you do it? Could you even come up with good evidence to show that the Great Pumpkin probably doesn’t exist? I can’t. But if you can’t, does that mean that you should see the issue as an open question, that you should regard it as a 50/50 proposition? No. Linus is a kid, but suppose he had to enter the real world, grow up, and go off to college. What would you think about him if he still believed in the Great Pumpkin when he was 32? What would you think if you arrived at college and found that your new classmate believed in the Great Pumpkin? What is the moral of this story?

Most of us cannot give strong evidence that the Great Pumpkin doesn’t exist, but we would regard anyone who thinks that it is a completely open question as much too gullible. Of course, we don’t encounter adults who believe in the Great Pumpkin. But we all encounter people who make some claim that seems implausible. Then, instead of building a positive case to support their claim, they suggest that since we can’t show that it’s wrong, it is probably true. We all have heard the refrain: “Well, you can’t show I’m wrong…”

This fallacy often occurs regarding claims of “things unseen,” such as elusive creatures like Bigfoot, or behind the scenes government activities and other conspiracies. When claims are about something being kept secret, it is easy to see why this fallacy would occur frequently.

Burden of Proof

When you make a claim that everyone agrees is true (e.g., that on earth the sun rises in the east and sets in the west), you don’t need to do much to build a case for it. When everybody already thinks something is so, you don’t need much in terms of evidence to show that it is. It is good for you to have reasons, but if you are never called to defend the claim, you won’t have much use for them. But if you make a surprising, controversial, or implausible claim (e.g., that several students have been abducted from campus by Martians), then things are a bit different. Now you have a strong responsibility to give reasons for your claim, and you are for sure going to be called to account for those reasons. The more implausible the claim, the heavier your burden of proof becomes. So, the fact that you can’t produce evidence that the Great Pumpkin does not exist gives you absolutely no reason to think that it really does.

When someone defends a view by pointing out that you can’t show that it’s false, they are committing the fallacy of appeal to ignorance. The fact that you are ignorant (don’t know) of evidence that would show they are wrong does not mean they are right. This is a fallacy of irrelevance, since the fact that I cannot show that some claim is false is not relevant to showing that it is true.

When someone makes a surprising claim, then adds, “Well, you can’t show that I’m wrong,” they are unfairly shifting the burden of proof to you. We often are in no position to prove that their claim is false. For example, if someone makes a claim about aliens from outer space infiltrating our critical reasoning course, we cannot prove that there haven’t been any. How could we? But the claim is implausible, and until someone gives us reasons to believe it, it’s reasonable to believe that it is false. This is worth repeating. The reasonable attitude here is not complete open-mindedness. It is not sensible to conclude that it’s a 50/50 proposition that creatures from outer space are stalking our campus. Until we are given some reason to believe this claim, it is much more reasonable suppose that it is false.

There are many cases like this. You are not now in any position to show that the Great Pumpkin doesn’t exist. But if you went around thinking that it was a 50/50 proposition that there was a Great Pumpkin, people would have serious doubts about you (and well they should). Of course, most of us aren’t worried about the Great Pumpkin. The fallacy is worth studying because there are many other, less obvious, cases of the same sort. In short: absence of evidence that X is false is not evidence that X is true. The fact that we cannot cite conclusive evidence for our view that there is not a Great Pumpkin is not evidence that there is a Great Pumpkin.

Note

An appeal to ignorance does not involve saying that someone else is ignorant, misinformed, or just plain dumb. The word ‘ignorance’ has a special meaning here. Someone commits the fallacy of an appeal to ignorance when they suggest that the fact that they haven’t been shown to be wrong is somehow evidence that they are right.

Positive vs. Negative Claims

Let’s call a claim that there are Xs a positive existence claim and a claim that there are not any Xs a negative existence claim. To show that a positive existence claim is true, it suffices to point to an example of X. If a biologist claims that they have discovered some new, unsuspected strain of virus, they can prove their case by producing a sample of it and allowing other scientists to test it.

But it can be very difficult to prove that a negative existence claim is false, particularly if it says that there are no Xs anywhere at all. For example, you cannot really look everywhere to determine that there are no Xs; you cannot look everywhere and then report that there wasn’t a Great Pumpkin anywhere you looked. Nevertheless, the claim that there is one is implausible; no credible witnesses have seen it, and science gives us no reason for believing in it.

Open-mindedness

Open-mindedness is a good trait to have, but it does not require us to seriously entertain any claim that comes down the pike. It does require us to remain ready to reevaluate any of our beliefs if new evidence or arguments come along and being willing to change our beliefs if the evidence requires it. But that doesn’t mean having such an open mind that you consider everything everybody says a serious possibility.

Implausible and Novel Claims can be True

A surprising, controversial, or implausible claim may, of course, turn out to be true. When great breakthroughs in science, medicine, technology and other fields were first announced, they often seemed implausible. History presents a sorry record of discoveries the society of the time wasn’t ready to accept; for example, the Catholic Church required Galileo to renounce his claim that the Earth moved around the Sun.

It is extremely important that such ideas be given a fair and open hearing, and that they be accepted if the evidence supports them. But this does not mean that every time someone comes up with a new idea it should be taken as seriously as ideas that are already supported by mountains of evidence. When the first vaccines were developed, the burden of proof was on those who developed them to show that they worked. In fact, they shouldered this burden and provided evidence to back up their claims. But it would have been rash for someone to have gotten an injection just because someone in a lab coat handed them a needle syringe and offered the comforting words: “Well, you can’t prove that it won’t work.” And for the record, the burden of proof is now on the people claiming that vaccines are dangerous and cause side effects like autism.

Reserving Judgment

In many cases the evidence on both sides of an issue is inconclusive. In such cases, it is best to suspend belief, to refuse to conclude that either side is correct. For example, there is not much strong evidence on either side of the claim that there is life on other planets. Of course, we may eventually find evidence that settles the matter (this would be much easier if there were extraterrestrial life and some of it showed up around here). But until then, it is reasonable to conclude that you just don’t have enough to go on, and so you just don’t know.

We are often in no position to disprove a surprising or implausible claim. But it isn’t our responsibility to do so. Our failure to supply evidence against the claim does not somehow provide evidence for it. If someone else wants to convince others that a novel claim is true, it is up to them to provide evidence for it.

Burden of Proof and the Law

An extremely important part of our legal system is that a defendant is presumed innocent until proven guilty. This means that the burden of proof is not on the defendant to show that he or she is innocent. The burden is on the prosecutor to show that the defendant is guilty. This makes sense, because the burden of proof is on the person who makes a claim (the claim here being that the defendant is guilty) rather than on the person on the other side.

To say that the defendant is presumed innocent is just another way of saying that we can’t use an appeal to ignorance to convict someone. We can’t argue, “well, they can’t show they didn’t do it, e.g., they don’t have an alibi, so they must be guilty.”

We typically have a higher standard for the burden of proof in the courtroom than in daily life, because the costs of mistakes are so high. In many cases, especially criminal cases, we require that the evidence be clear beyond any reasonable doubt. Because the burden of proof lies with the state, a defendant is under no compulsion to testify at all. But juries, being human, often see failure to testify as some indication that the defendant is guilty. And this gives us the last important lesson about burden of proof. The greater the consequences of a decision, the greater the burden of proof is on the issue.

Exercises

1. Appeals to ignorance do not just arise with the Great Pumpkin. It is common for companies to argue that it has yet to be demonstrated that certain things are dangerous (e.g., smoking cigarettes, nuclear power plants, certain sorts of landfills, and toxic waste dumps), and so we should continue to build or manufacture such things. Do such cases involve a fallacious appeal to ignorance (the answer may be different in different cases)?
2. Conspiracy theories often trade on appeals to ignorance. Since investigators haven’t been able to show that something isn’t the case, it is suggested, there is strong reason to think that it is. Give an example of this (it can be one you have read about or one you invented).
3. Appeals to authority are sometimes legitimate. Can you think of any special circumstances where an appeal to ignorance might be legitimate?
4. When the people on both sides of an issue are making claims that aren’t at all obvious, then the burden of proof falls on them both. Give an example of this sort.

And then, of course, we have these two claims:

5. It’s reasonable to conclude that God exists. After all, no one has ever shown that there isn’t a God.
6. It’s reasonable to conclude that God does not exist. After all, no one has ever shown that there is a God.