1984 - Pg 204

   "But it did exist! It does exist! It exists in memory. I remember it. You remember it."
   "I do not remember it," said O'Brien.
   Winston's heart sank. That was doublethink. He had a feeling of deadly helplessness. If he could have been certain that O'Brien was lying, it would not have seemed to matter. But it was perfectly possible that O'Brien had really forgotten the photograph. And if so, then already he would have forgotten his denial of remembering it, and forgotten the act of forgetting. How could one be sure that it was simply trickery? Perhaps that lunatic dislocation in the mind could really happen: the was the thought that defeated him.