Winston is stubborn in believing that the Party will succeed if they continue their lies and their cruelty. He believes that the Party will never overcome everything/everyone in the world. This shows Winston's stubbornness. Up until a certain point, Winston stands firm against O'Brien's questions, but he comes to a point where he can no longer go on under the relentless questioning.

Winston: 'It is impossible to found a civilization on fear and hatred and cruelty. It would never endure.'
O'Brien: 'Why not?'
Winston: 'It would have no vitality. It would disintegrate. It would commit suicide.'
O'Brien: 'Nonsense. You are under the impression that hatred is more exhausting than love. Why should it be? And if it were, what difference would that make? Suppose that we choose to wear ourselves out faster. Suppose that we quicken the tempo of human life till men are senile at thirty. Still what difference would it make? Can you not understand that the death of the individual is not death? The party is immortal.'
Winston: 'I don't know -- I don't care. Somehow you will fail. Something will defeat you. Life will defeat you.'
O'Brien: 'We control life, Winston, at all its levels. You are imagining that there is something called human nature which will be outraged by what we do and will turn against us. But we create human nature. Men are infinitely malleable. Or perhaps you have returned to your old idea that the proletarians or the slaves will arise and overthrow us. Put it out of your mind. They are helpless, like the animals. Humanity is the Party. The others are outside -- irrelevant.'
Winston: 'I don't care. In the end they will beat you. Sooner or later they will see you for what you are, and then they will tear you to pieces.'
O'Brien: 'Do you see any evidence that that is happening? Or any reason why it should?'
Winston: 'No. I believe it. I know that you will fail. There is something in the universe -- I don't know, some spirit, some principle -- that you will never overcome.'
O'Brien: 'Do you believe in God, Winston?'
Winston: 'No.'
O'Brien: 'Then what is it, this principle that will defeat us?'