In Book 3, Ch. 2 O'Brien shows Winston the "mutability of the past." Explain the incident that he uses to demonstrate this concept to Winston?
At this moment in Book 3, Chapter 2, O'Brien is torturing Winston physically and psychologically in room 101. During this incident of torture, O'Brien is overpowering Winston. In a physical sense, Winston is weakened by pain and fear as O'Brien dials up the severity of his pain to 40, and in a psychological sense, Winston is broken and confused as O'Brien forces him to accept "the mutability of the past." If Winston does not accept that the past is changeable, he will experience the kind of pain that he fears the most, which is what makes room 101 so terrifying.
During this experience of torture, O'Brien shows Winston a photograph, a document that contains undeniable visual proof that gives Winston reassurance: "It was a photograph, and there was no question of its identity. It was THE photograph. It was another copy of the photograph of Jones, Aaronson, and Rutherford at the party function in New York, which he had chanced upon eleven years ago and promptly destroyed." When O'Brien tosses this photograph into the memory hole and denies that the photograph ever existed, he is presenting Winston with "the mutability of the past." If Winston in turn denies that the past is mutable, he will likely not survive this session of torture.
Eleven years before the events in this chapter, Winston had seen a picture that proved that something that the Party said was false. In this chapter, O'Brien shows Winston a copy of that picture. He then throws it in the incinerator and tells Winston that the picture never existed. Winston says that he remembers the picture and therefore it existed. O'Brien says that he does not remember the picture and therefore it does not exist.
By doing this, O'Brien shows the power of doublethink and how that can change the past. By being able to truly forget the picture and to forget that he had forgotten, O'Brien can change the past. Once everyone forgets what really happened, the past has changed and a false "memory" becomes the actual "truth" because that is what everyone remembers.