Big Brother controls history in 1984. Party leaders choose what to edit on a daily basis for their purposes. Even the trace of human's existence is wiped out as are any extra scraps of paper. That means there are no pieces of evidence to look back on to try to know how it used to be.
The threat of the Thought Police seems to keep people in check. Winston worries that his thoughts will be seen. Winston is excited about his diary, but afraid it will be found. They work hard not to say anything against the Party to each other for fear that they will be heard or spied on through a telescreen.
Fear is a powerful motivator in the wrong hands. 1984 is a novel which clearly illustrates that rebellion is next to impossible when a leader rules by dishing out fear.