The Ministry of Truth is involved with news media, entertainment, the fine arts and educational books. Its purpose is to rewrite history and change the facts to fit Party doctrine for propaganda effect. For example, if Big Brother makes a prediction that turns out to be wrong, the employees of the Ministry of Truth go back and rewrite the prediction so that any prediction Big Brother previously made is accurate. This is the "how" of the Ministry of Truth's existence. Within the novel Orwell elaborates that the deeper reason for its existence is to maintain the illusion that the Party is absolute. It cannot ever seem to change its mind (if, for instance, they perform one of their constant changes regarding enemies during war) or make a mistake (firing an official or making a grossly misjudged supply prediction), for that would imply weakness and to maintain power the Party must seem eternally right and strong.
