The Burden of Proof and Arguments from Ignorance

Sometimes people engaged in a disagreement try to get out of providing reasons in support of their view by claiming that their opponent has the burden of proof. That is, they claim that their own position is the default position and doesn’t need to be supported by arguments, and that it is up to their opponent to prove them wrong.

Here’s an example. Suppose I’m having a dispute with a pesticide manufacturer about whether using NoMoreBugs on your tomato plants is bad for your health. And suppose the manufacturer says “NoMoreBugs is safe. You have no evidence that it’s not! We’re going to keep selling NoMoreBugs.”

Hang on a minute. Why is it up to me to show that NoMoreBugs is unsafe, rather than up to the manufacturer to provide evidence that it is safe? The manufacturer’s argument is pretty much:

P1. You haven’t proved that NoMoreBugs is unsafe.
                                   
C. NoMoreBugs is safe.

When someone argues that because there’s no proof that their claim is false, therefore it’s true, they are committing a fallacy called an Argument from Ignorance – arguing that since we don’t know that the claim is false, it must therefore be true.

There are all kinds of claims that are extremely likely to be false even though we can’t prove them false.The fact that we can’t prove something false is not enough reason for us to believe it.  We can’t prove that it’s false that 50% of the people in this class are extraterrestrials pretending to be humans. That doesn’t mean that we have reason to believe that 50% of us are extra-terrestrials pretending to be humans.

Sometimes arguments from ignorance are a bit more subtle. Suppose a tabloid newspaper publishes some scandalous allegations – “Stop Press! Winston Involved In Threesome With Jacinda and  Clarke!” And then suppose the next issue contains a piece about how Winston and Jacinda and Clark have failed to deny these allegations, which concludes that the allegations must therefore be true. This is an appeal to ignorance. The fact that no one has produced evidence that the claim is false is not enough reason to believe it’s true. There are lots of reasons why Winston and Jacinda and Clarke might not deny the claim even if it’s false. For example, “NZ PM and Deputy PM deny allegations…” might make a headline in a reputable mainstream newspaper, thus distributing the unsubstantiated allegations to a wider audience, and that might be something the people involved would rather avoid.

Arguments from ignorance involve illegitimate attempts to shift the burden of proof. 

In a courtroom, the burden of proof rests with the prosecution. The defendant is considered innocent until he or she is proven guilty. It’s up to the prosecution to prove the defendant guilty. The defense does not have to prove the defendant innocent – they only need to show that there are holes in the prosecution’s arguments; that there is a reasonable doubt about whether the defendant is guilty.

In the courtroom situation, we know who has the burden of proof – it’s built into our justice system. But in other cases, it’s not so clear, and there’s no one story which applies across the board about what makes it so that the burden of proof is on one person rather than another.

If I’m claiming something which conflicts with what is generally accepted, then the burden of proof is on me – I shouldn’t expect my unorthodox view to be accepted by others unless I provide evidence or arguments for it. When Copernicus claimed that the earth goes round the sun rather than vice versa, he needed to supply evidence – the burden of proof was on him – because the traditional view was that the sun goes round the earth. But if I now get into a dispute with someone about this question – I claim the earth goes round the sun, they claim the sun goes round the earth – then the burden of proof is on them – nowadays, the general view is that the earth goes round the sun.

There are other considerations which are relevant to who has the burden of proof. One of them is safety. On the whole, if people’s safety is in question, the burden of proof is on the people arguing that the thing is safe to prove that it is. Why? The consequences of being too careful are much less bad than the consequences of not being careful enough.

But note that this can open the way to malicious, unsubstantiated objections. If a particular medication has only been in use for 10 years, there is no way to conclusively prove that it has no long-term side-effects, and it would be unreasonable to require a drug company to prove that it has none if there is no particular reason to suppose that it has.

Another consideration is whether your view conflicts with a consensus amongst authorities. Suppose that all the most-respected theorists in the field of education agree that exams are not a good way of testing students’ knowledge and abilities – supposed the received view in education is that exam results mostly reflect how good the student’s exam-sitting technique is, and that internal assessment is much more useful. Then, if I want my course to be assessed entirely by exam because I think that actually making people write down a whole lot of information under time pressure actually is the best way to tell how much they’ve learned, it’s reasonable that the burden of proof should be on me – if I’m flying in the face of what all the educational authorities agree on, I should make a case for my view – I should present arguments.

So there are these are three kinds of cases when the burden of proof is on you:

1. When you are making a claim that conflicts with common wisdom,
2. when you are claiming that some product or practice is safe, and
3. when you are making a claim that conflicts with expert consensus.

Almost always, in fact, you should be willing to provide evidence in support of your claims – why should anyone believe you, about any controversial matter, if you don’t provide reasons in support of your view? But these are three kinds of case in which it’s particularly important to be able to provide reasons in support of your view.

Note that in saying that the burden of proof is on you if what you are claiming conflicts with what is commonsensically believed, I am not saying that the fact that most people believe something is a good enough reason for you to believe it too. It never is. For example, it’s traditional to think that we don’t need to be nearly as considerate of the needs of animals as we should be of the needs of humans, and most people still believe that’s true. But I shouldn’t believe that, and you shouldn’t believe that, just because most people do or because it’s a belief with a long and illustrious pedigree. That most people believe it doesn’t show that it’s true. It might, however, put the burden of proof on the person who disagrees with it – if they want to change people’s minds, they will need to provide a compelling argument.