> You can't prove that would-be terrorists weren't dissuaded by the knowledge that PRISM existed.
This is the tiger repelling rock theory of anti-terrorism. I have a rock that repels tigers. There haven't been any tiger attacks when I'm around, so it must work.
> I have a rock that repels tigers. There haven't been any tiger attacks when I'm around, so it must work.
Behind the scenes, the man with the rock is using it to kill every single kitten he sees when he's alone. He can't prove how useful the rock is without admitting to all the senseless murder of innocent bystanders.
The details of my anti-tiger operations are highly classified; whoever leaked this information to you will be vilified and hounded around the globe for the rest of their life.
This is the tiger repelling rock theory of anti-terrorism. I have a rock that repels tigers. There haven't been any tiger attacks when I'm around, so it must work.