Sometimes you can prove an alleged event did happen, but you generally can’t prove something did not happen. For example, if police have clear video footage of a crime in progress, several direct witnesses, and DNA evidence too, you can say they proved the defendant did the crime. But if your neighbor says an angel visited him in his bedroom at night, and there were no witnesses or physical traces left behind, you can’t prove it didn’t happen. All you can say for sure is that you don’t have any evidence of it happening.

So if you want to drive a political opponent crazy, allege that he or she did something evil, provide no direct evidence, and force them to do the impossible: Prove it didn’t happen.

Consider the Russian collusion investigation. We have seen no conclusive evidence that President Trump colluded with Russia to win the election. But the mere existence of an investigation into the allegations, along with lots of “Russia, Russia, Russia” news coverage on tangential topics, primes us to think “Where there is this much smoke, there must be fire.” To escape this trap, President Trump would need to do the impossible. He would need to prove he didn’t collude with Russia in some secret way that left no evidence behind. And you can’t prove a negative, as the saying goes.

So how do you defend yourself when you can’t prove something didn’t happen? One way is to turn the same trick against your attackers. An emerging story this week is that Republican Representative Devin Nunes wrote a secret memo detailing alleged various abuses to the FISA system that are somehow related to the same FBI and DOJ players who worked on the Russian collusion case. The public hasn’t seen the memo, politicians are barred from discussing it in detail, and there’s a good chance we will never see it in its full context. The longer we hear about the secret memo without knowing its contents, and the more speculation that gathers around it, the more the public will be thinking “Where there is this much smoke, there must be fire.”

You can’t prove a negative. But you can return the favor and put your opponent in the same position.