The Fourth Amendment to the US Constitution guarantees this explicitly: no one should be subjected to “unreasonable searches or seizures” without “probable cause”. Which is why there was such a tumultuous public uproar in the United States when the NSA’s mass surveillance programme was exposed. But the British unwritten constitution has historically made the same assumption: that no one should be prey to investigation of his day-to-day activities without the due process that law enforcement agencies are required to follow. And further, such processes should be applied to individual cases, not on an indiscriminate basis to great cohorts of the population (let alone the whole of it). This is a rule that is fundamental to a free society. Without it, there can be literally no limit to the harassment, interrogation or persecution of a citizen on no good grounds at all – or worse, on the basis of political prejudice.