Also Known As: Appeal to Ignorance, Ad Ignorantiam
Burden of Proof is a fallacy in which the burden of proof is placed on the wrong side. One version occurs when a lack of evidence for side A is taken as evidence for side B when the burden of proof rests on side B. This is bad reasoning because side B is the one with the obligation to make their case. This fallacy looks like this:
Premise 1: Claim X is presented by A and the burden of proof rests on B.
Conclusion: Side B claims that X is false because there is no proof for X.
In some debates, one side will have the burden of proof. This side is obligated to provide evidence for its position. The claim of the other side, the one that does not bear the burden of proof, is (usually) assumed to be true unless proven otherwise.
It can sometimes be difficult, even in good faith debates, to determine which side, if any, the burden of proof rests on. If this cannot be done in good faith, the reasonable thing to place the burden on all sides. In terms of guidance, there are some suggestions about who should have the burden of proof.
Intuitively, the side that is less plausible should have the burden of proof. For example, if I claimed to have run a marathon in under two hours, the burden of proof would be on me and not on the people who deny my claim. But the question of which side has less initial plausibility is also a matter of debate, and this would lead to another question of who should have the burden of proof.
Ease of proof (or disproof) can also be used to decide who has the burden. If one claim can be easily proven and its denial would be extremely difficult to prove, then the burden should be on the side that can more easily prove its claim. For example, if someone claims that ghosts exist on earth, it would be easier to find a single ghost and prove the claim than it would be to completely examine the entire planet to prove there are no ghosts.
In some cases, the burden of proof is set by context. For example, in American law, a person is assumed to be innocent until proven guilty. So, the burden of proof is (in theory) on the prosecution. As another example, in debate, the burden of proof is placed on the affirmative team.
As a final example, in most cases, the burden of proof rests on those who claim something unusual exists (such as Bigfoot, psychic powers, metaphysical universals, and ghosts). But what counts as unusual can be debated.
While the burden of proof can be wrongly placed in good faith, a bad faith tactic is to intentionally place the burden of proof wrongly on the opposing side. This can provide a significant advantage since the side stuck with the burden of proof will be seen as needing to prove their claim while the person arguing in bad faith will be assumed correct until proven otherwise.
Another variant of this fallacy is to infer that a claim is true because there is no evidence against it. A common name for this is an Appeal to Ignorance. The form of this fallacy is:
Premise 1: There is no evidence against claim C.
Conclusion: Claim C is true.
The mistake in this reasoning is that a lack of evidence against a claim does not serve as positive evidence for that claim. This fallacy gets its appeal because assessing a claim does involve considering evidence against it. And it is tempting to think that a lack of evidence against a claim is thus evidence for a claim. If there is good evidence for a claim, then a lack of evidence against it would be a plus in favor of the claim. This assumes that possible evidence against the claim has been properly investigated.
It would also be a fallacy to infer that a lack of evidence for a claim disproves the claim. As the saying goes, absence of evidence is not evidence of absence. This reasoning has the following form:
Premise 1: There is no evidence for claim C.
Conclusion: Claim C is false.
It is tempting to think that a lack of evidence for a claim is evidence against it. However, while a lack of evidence does entail that you should not accept the claim, not accepting a claim is different from rejecting a claim. If there is no evidence for a claim, but also no evidence against it, then the rational thing to do is suspend judgment.
Defense: One defense against the Burden of Proof fallacy is to carefully consider which side needs to do the proving. This might end up being all the sides. For the variants, the main defense is to keep in mind that a lack of evidence for a claim is not proof that the claim is false and that the lack of evidence against a claim is not proof it is true. Only evidence for a claim is evidence for a claim and only evidence against a claim is evidence against a claim.
Bill: “I think that we should invest more money in expanding the interstate system.”
Jill: “I think that would be a bad idea, considering the state of the treasury.”
Bill: How can anyone be against highway improvements?”
Bill: “I think that some people have psychic powers.”
Jill: “What is your proof?”
Bill: “No one has been able to prove that people do not have psychic powers.”
“You cannot prove that God does not exist, so He does.”
“There is no evidence that the actor was lying about being attacked, so these claims that he made it up to get attention must be lies.”