How does O’Brien argue that the Party’s memory is always correct?

During Winston's torture, O'Brien turns a lever, and Winston is overcome with pain. O'Brien continues talking to Winston, warning him that the dial goes all the way up to 100. He explains to Winston that he is taking the trouble to truly work with him to correct his mental derangement and defective memory. He begins by asking Winston what country Oceania is at war with, and what country Oceania has always been at war with. He then moves on to the newspaper clipping Winston found regarding Jones, Aaronson and Rutherford. He produces a copy of the photograph, places it in a memory hole, and explains that it never existed. He forces Winston to understand that the past does not exist in space, and therefore only exists in records and in human memory. In controlling these two things, the Party controls the past. Winston argues that the Party has not controlled his memory, and O'Brien counters that in fact, Winston has not controlled his own memory. He is at the Ministry of Love because he "failed in humanity, in self-discipline...would not make the act of submission which is the price of sanity." O'Brien explains, "Reality exists in the human mind, and nowhere else."