The totalitarian regime in 1984 encourages the populace to practise doublethink: the ability to accept as true what you know to be false. For example, last week you knew that Party Member X was your next-door neighbour and loyal to the precepts of Ingsoc. However, this week the government has decided that such a person never existed - he is an unperson. Doublethink deprecates your own memory as against the memory of the system: if the party says that the person never existed, then who are you to say that he did? This is internalised, so not only do you not claim to others that this person existed: you do not even allow yourself to believe it. This supression of your own mental processes effectively gives the state mind control.
Of course, 1984 is a work of fiction, but this concept has chilling analogues in our own age, especially with the interpretation of history. For example, during the Gulf War, were we allowed to remember the historical fact that Saddam Hussein was once an ally of the United States, who provided him with weapons?