By without history, Winston has no reference, no reason to think that he is right. For reality is no longer with the individual, but with the society. If I were to lean against a wall and say that I was pushing it, the wall would not move. But if society thought that the wall was moving, then it would be immaterial if the wall was actually moving. The definition of a moving has now been changed to the simple act of leaning against a wall.
Reality is no longer a constant.
That is why double think is needed. Winston can't write because of the deterioration of language, known in 1984 as newspeak. By truncating words, the party is able to limit thought. For people associate their thoughts only with words, and without words to define thought, the thought is meaningless. Without thought, the people follow.