Winston contemplates a lot about how the Party has completely rewritten history and regularly does it--and he knows this because it is apart of his job. And there is the idea that we don't know exactly what is true and what is not true. He realizes that without a record and no memory of the people, proving whether something happened or not is really tricky.
As a history teacher, we understand that there is bias that is inevitable in history writing. But this story takes that bias to the max! Everyone knows that the history is written really by the victors and this book really begs the question, how much of that can we really believe?