It is important to keep in mind that when security is working, you don't see any evidence of it. We don't actually know how many terrorist attacks have been stopped by this surveillance. It may well be a lot. It wouldn't surprise me at all to learn that many have been. Which is why it's so important that we do not ground criticisms of surveillance in arguments about efficacy. Surveillance is effective, or at least, it can be. Surveillance is wrong because it concentrates too much power in the hands of the government.