His love for Julia. He believes that even though he has utterly betrayed and renounced himself and everything he believes in, he has still held on to his humanity because he never stopped loving her, even under torture. This makes his final capitulation even more horrifying and symbolic. When O'Brien presents him with the prospect of having his face eaten by rats (the thing he fears most), he "saves" himself by screaming "Do it to Julia!" This illustrates the darkest, and perhaps most ironic part of the novel: by saying these words Winston saves his physical life, but O'Brien has succeeded in destroying everything in Winston that made him human.