Three Conceptions of the Burden of Proof
In rational discourse, the burden of proof is the obligation on a person to provide evidence for their position.
I. We might think that the burden of proof rests with the person asserting a claim. But consider:
(i) Suppose A asserts claim P, and afterwards, B asserts claim ~P. Under this conception, A bears the burden of proof.
(ii) B asserts claim ~P, and afterwards, A asserts claim P. In this case, B bears the burden of proof.
Except for the order of speaking, both cases are functionally equivalent; yet the burden switches from A to B. But surely the burden of proof does not depend on something as arbitrary as the order in which people speak.
One might object that both A and B bear the burden. This could be true sometimes. But sometimes only one of A or B bears the burden, and this conception fails to account for such cases. (For example, when P is the statement “The sky is blue.”)
II. A less naive conception is that the burden of proof rests with the person asserting a positive claim. But consider:
(iii) A asserts positive claim P, so that A bears the burden of proof.
(iv) A asserts negative claim ~(~P), so that A does not bear the burden of proof.
But since P and ~(~P) are equivalent, we have that A both does and does not bear the burden of proof. Since this is a contradiction, this conception must be flawed.
Now, it’s certainly true that the burden sometimes does rest with the person making the positive claim; if A claims that UFOs exist, then A bears the burden of proof. But there are easy counterexamples: if B makes the positive claim that the sky is blue, then surely the burden rests with those who would disagree.
III. The correct conception, it seems to me, is that the burden of proof is determined by common sense and expert consensus. If someone asserts a claim that contradicts common sense or expert consensus, then they bear the burden of proof. This conception captures what is correct in the other conceptions, and avoids their mistakes. For example, the person who claims that UFOs exist bears the burden of proof, not because it’s a positive claim, but because it contradicts common sense and expert knowledge. The person who would deny that the sky is blue bears the burden because they are contradicting common sense.
To be sure, common sense and expert consensus are imperfect. This is why the burden of proof is defeasible: if it turns out that common sense is defective, or if the experts are unreliable, then in those cases the burden has to be determined by other considerations.
In sum, simplistic formulas for determining the burden of proof are mistaken. Instead, establishing the burden requires making arguments and judgments about common sense and expert consensus.