“[A] structural approach, which focuses on preserving an overall balance between state control and citizen autonomy, seems to me more appropriate for evaluating mass surveillance programs such as the NSA’s … . [T]he appropriate question is whether the creation of a system of surveillance perilously alters that balance too far in the direction of government control, whether or not we have problems with the current use of that system. We might imagine a system of compulsory cameras installed in homes, activated only by warrant, being used with scrupulous respect for the law over many years. The problem is that such an architecture of surveillance, once established, would be difficult to dismantle, and prove too potent a tool of control if it ever fell into the hands of people who—whether through panic, malice, or a misguided confidence in their own ability to secretly judge the public good—would seek to use it against us.”