In book 3 chapter 2 of 1984, Winston is in his cell drifting in and out of consciousness. He confesses to all kinds of things, even things he's never done. And hallucinations begin. He imagines O'Brien is there.
He's now being hooked to a machine whose pain threshold can be dialed from 0 to 100. His mind is being reprogrammed by the party to believe that history has never been revised. When O'Brien holds up four fingers, he's really holding up five. O'Brien explains that it is intolerable to the party to allow individual thought to exist. They must torture and interrogate a person until everything is dead inside, and that person is free of emotions. Readers are now seeing O'Brien for who he truly is.
He takes truth and untruth seriously. And the two men have diametrically opposed opinions about what is actually true. Maybe it was O'Brien's commitment to the truth that first drew Winston to him.
Winston and Julia have asserted that their private thoughts and memories are their own. O'Brien destroys that certainty by tearing down what Winston remembers to be true. O'Brien tries to get Winston to believe that being in a minority of one truly makes you crazy.
There is no truth, but the truth the party tells you. video.