Hmm, that's a good point. It is a problem. And a problem for both sides.
If X is the amount of utility coming out of surveillance, and you cannot know X, then you cannot argue that X = 0 or that X > Y (for any Y you want to pick, like downsides of surveillance) or that X < Y either.
Essentially it becomes impossible to rationally debate the issue on the basis of whether it is a net gain.
Which means you need to fall back on other forms of reasoning. A reasonable position is that freedoms shouldn't be sacrificed for something whose utility cannot be demonstrated. But that's an argument about what sorts of justifications are required for laws, not about how much utility the law would have.
If X is the amount of utility coming out of surveillance, and you cannot know X, then you cannot argue that X = 0 or that X > Y (for any Y you want to pick, like downsides of surveillance) or that X < Y either.
Essentially it becomes impossible to rationally debate the issue on the basis of whether it is a net gain.
Which means you need to fall back on other forms of reasoning. A reasonable position is that freedoms shouldn't be sacrificed for something whose utility cannot be demonstrated. But that's an argument about what sorts of justifications are required for laws, not about how much utility the law would have.