That's true, though I'm not speaking to that point at all. Rather, I wanted to prevent a debate following from someone attempting to make a point that's technic

That's true, though I'm not speaking to that point at all. Rather, I wanted to prevent a debate following from someone attempting to make a point that's technically indefensible but not necessarily sound. Expressing the same point without that kind of demand ("prove this isn't happening") would be fine. But I do agree with you - this can be applied to FISA courts' efficacy. But that's not entirely a rebuttal since it's equally valid in both cases. Saying that in response as a rebuttal would wrongly imply it's fair to demand a proof of the negative just because it's valid for FISA effectiveness as well.