"Doublethink means power of holding two contradictory belief's in one's mind simultaneously, and accepting both of them. The Party intellectual knows in which direction his memories must be altered; he therefore knows that he is playing tricks with reality; but by the exercise of doublethink he also satisfies himself that reality is not violated. The process has to be conscious, or it would not be carried out with sufficient precision, but it also has to be unconscious, or it would bring with it a feeling of falsity and hence guilt. Doublethink lies at the very heart of Ingsoc*, since the essential act of the Party is to use conscious deception while retaining the firmness of purpose that goes with complete honesty. To tell deliberate lies while genuinely believing in them, to forget any fact that has become inconvenient, and then, when it becomes necessary again, to draw back from oblivion for just so long as needed, to deny the existence of objective reality and all the while to take into account of the reality one denies--all this is indispensably necessary. Even in using the word doublethink it is necessary to exercise doublethink. For by using the word one admits that one is tampering with reality; by a fresh act of doublethink one erases this knowledge; and so on indefinitely, with the lie always one leap ahead of the truth. Ultimately it is by means of doublethink that the Party has been able--and may, for all we know, continue to be for thousands of years--to arrest the course of history."
-1984 by George Orwell