control of the past = control of the future and control of the present = control of the past

"Who controls the past," ran the Party slogan, "controls the future: who controls the present controls the past." This quote is an important idea in the book because it relates to the reach of the party's control. The party, as Winston says "It was not true... that the party had invented aeroplanes," but he says that you cannot prove that because the party has destroyed evidence. This idea says that if you alter the beliefs and evidence of what happened in the past people will begin to believe that fact and it begins to influence their future actions. It also says that if you control what people think, screening what people learn about in the present, you can alter their perceptions of the past. I think that this shows how deep the party is in the roots of society, so much so that they can alter and control peoples thoughts and what has happened in the past. The fact that they can change the past to their benefit is scary and shows that they have total control of almost every aspect of every persons life. This shows that they are very thorough because, although people have memories of pasts different from what the party says, all evidence backs up the party's side.

http://www.sparknotes.com/lit/1984/themes.html