I hit quite a snag the other day in the Critical Thinking class that I am privileged to teach at a community college. By the way, I dislike that word “teach”; I prefer to think of what I do as a modest attempt to nudge students into learning by themselves. Anyway, back to the snag:

A student took umbrage at a comment made by author Michael Specter in his book Denialism. Specter wrote that there was no evidence that certain residual pesticides in our food had ever harmed anyone. The student asked, “Where is there no evidence?” I tried to explain why that question was absurd, but ran into considerable difficulty articulating my point. I hope to do better now because I fear that many people do not have a clear grasp of a particular principle of critical thinking.

It’s up to the claimant to prove a claim is true.

If you claim a certain pesticide is harmful — or even potentially harmful — it’s up to you to provide positive evidence of possible harm. It’s reasonable to demand that the pesticide undergo rigorous testing, but it is not reasonable to claim that the pesticide is harmful just because there is no evidence that it is not harmful. That kind of negative evidence is impossible to come by; therefore, it is irrational to demand it. As is often said, you cannot prove a negative. It’s impossible to prove that something does not exist. No one, for instance, can prove that ghosts are not real. Likewise, it’s impossible to prove that potential harm does not exist. No one can guarantee that any product or anything else is absolutely safe under all circumstances. It’s unreasonable to ask a manufacturer to prove that Substance X will never be harmful to anyone. Everything on earth is potentially harmful to somebody. That doesn’t stop us from driving cars, swimming in the ocean, drinking beer, or eating peanuts.

You’ve probably heard the adage, “absence of evidence is not evidence of absence.” In a sense, that is true. If there is no verifiable evidence that Substance X causes cancer, Substance X might very well still cause cancer, but we can’t make a positive claim that it does cause cancer without proof that it really does. On the other hand, we would be amiss to claim that Substance X could never cause cancer because the possibility exists, however remote, that the substance could be carcinogenic to some living organism on the planet.

By asking “Where is there no evidence?” the student demanded that Michael Specter reveal something that does not exist. The implied argument is that if Specter cannot do the impossible, then we can conclude that Specter’s statement is probably false. The student is shirking his responsibility to supply positive evidence, while demanding that Specter prove a negative. Specter did not say that the residual pesticides in question have never harmed anyone; he simply pointed out that there was no evidence that they ever had. If the student has evidence to the contrary, that the residual pesticides have indeed harmed someone, the student needs to present that evidence.