People should stop saying "You can't prove a negative" as if it were some sort of logical law. It has nothing to do with logic, you can prove negatives.
One popular meme I see floating around the Twitterverse is the assertion that the burden of proof is always on the positive claim, or that ‘you can’t prove a negative’ (we’ve all seen or said this plenty of times). As a rule of thumb, this does quite nicely. We more often than not run into positive claims than negative claims, so it rarely comes up. But nearly nobody on Twitter who uses the burden of proof in debate understands when it properly applies. This leads them to believe that proponents of negative claims are intellectually dishonest, as it allegedly saddles them with an impossible burden of proof.
The assigning of the burden of proof relies on where the claim sits in relation to known background information. If I make a claim that is consistent with everything we know about the world, there is little to no burden on me. The work has already been done. The likelihood of what I’ve said being true is so high that we don’t even talk about burdens at all. In Darwin’s day, the prevailing explanation for the complexity of life was intelligent design. It was his burden of proof to argue this was not the case. Yet today the burden is reversed, because intelligent design is inconsistent with what is known about the origin of biological complexity.
But it might be objected that this is just “one positive claim (evolution is true) against another positive claim (design is true). What we’re concerned about is the impracticality of proving negative claims, claims to non-existence”. Superficially, this is the case. But notice that the claim ‘evolution is true’ is a denial of the claim that ‘design is true’. One can rewrite the positive claim ‘evolution is true’ as the negative claim ‘non-evolutionary explanations are false’. The same evidence supports both claims. I sense many will be unsatisfied with this answer, so let’s use another example.
If I claim to have $20 in my pocket, you’ve really got no reason to disbelieve me on that information alone since it’s trivial. A mere $20 isn’t an unusual amount of money to have in your pocket, so the burden is minor or nil. But if I make the (positive) claim that I have $20 in my pocket and a leprechaun loaned it to me then you have cause to raise an eyebrow, and ask further questions. The existence of a money-lending leprechaun is so out of whack with everything else we know about the universe that it demands special explanation, and the burden now applies.
Now consider the converse. I make the (negative) claim that money-lending leprechauns don’t exist. This is consistent with known background information. As such, there is little to no burden to demonstrate it. It’s a trivially true claim against the background of metaphysical naturalism, the philosophical position that there are no supernatural entities of any kind. We are justified in provisionally accepting this metaphysical position because we’ve never seen evidence to its contrary, and all the evidence we do have supports it. If supernatural entities like leprechauns existed it would require a wholesale revision of everything we think we know about the universe, and the probability of this occurring is infinitesimal given the wealth of evidence we have for the universe operating how we currently believe it does.
In this way, the burden of proof is related to the informal fallacy of ‘argument from ignorance’. It’s important to understand when this charge is actually a fallacy, and when it is not. Whether it sticks will depend on who has the burden of proof, which will depend on known background information. When intelligent design proponents claim that everything in the genome has a designed function and we just haven’t found it yet, that’s a fallacy. When the evolutionary biologist claims that there has never been found an instance of an intelligently designed biological organism, therefore ID is false, it is not a fallacy. The evidence is clearly on one side.
Now a hard-nosed skeptic will perhaps reply that science isn’t complete, and we don’t know everything, so how can we categorically rule out the existence of things? But this is again to confuse ontology with epistemology, to treat knowledge as absolute truth rather than a provisional assessment of truth.
Topic: You can prove a negative