In the novel, the government has complete control and may censor anything it likes. The government decides what is true, and if it feels, it may change what is true and make sure all believe their new truth has always been true.
Take Winston's job at the Ministry of Truth. His job to go through documents and edit or change documents to match the current version of truth according to the government. Winston also knows that going against these accepted "truths" may mean disappearing without a trace. Therefore, if the government says 2+2=5, than it is true. Even if an individual knows it to be incorrect, he or she must accept it as true. For example, Winston starts to realize more and more that things put out by Big Brother are falsehoods and brazen lies, but he does not have the freedom to deny their claims, let alone protest against them.