“Do it to Julia!”

From George Orwell's 1984 [spoiler warning]:

‘By itself,’ he said, ‘pain is not always enough. There are occasions when a human being will stand out against pain, even to the point of death. But for everyone there is something unendurable — something that cannot be contemplated. Courage and cowardice are not involved. If you are falling from a height it is not cowardly to clutch at a rope. If you have come up from deep water it is not cowardly to fill your lungs with air. It is merely an instinct which cannot be destroyed. It is the same with the rats. For you, they are unendurable. They are a form of pressure that you cannot withstand. even if you wished to. You will do what is required of you.’

Winston Smith has been tortured and humiliated, he has lost his dignity, he will believe that 2+2=5, but has not betrayed Julia – until O'Brien exploits his ineradicable, instinctive weakness.