There was of course no way of knowing whether you were being watched at any given moment. How often, or on what system, the Thought Police plugged in on any individual wire was guesswork. It was even conceivable that they watched everybody all the time. But at any rate they could plug in your wire whenever they wanted to. You had to live—did live, from habit that became instinct—in the assumption that every sound you made was overheard, and, except in darkness, every movement scrutinized.
-- George Orwell, 1984 (1949)
The essence of it consists, then, in the centrality of the inspector’s situation, combined with the well known and most effectual contrivances for seeing without being seen ... You will please to observe, that though perhaps it is the most important point, that the persons to be inspected should always feel themselves as if under inspection, at least as standing a great chance of being so ... the greater chance there is, of a given person’s being at a given time actually under inspection, the more strong will be the persuasion—the more intense, if I may say so, the feeling, he has of his being so.