"According to the novel, doublethink is:
the power of holding two contradictory beliefs in one's mind simultaneously, and accepting both of them. ... 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 it back from oblivion for just so long as it is needed, to deny the existence of objective reality and all the while to take account of the reality which 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. (pages 35, 176-177)
As Orwell explains in the book, the Party could not protect its iron grip on power without degrading its people and exposing them to constant propaganda. Yet knowledge of this brutality and deception, even within the Party itself could lead to disgusted collapse of the state from within, as the Soviet Union later fell in the late 20th century. For this reason, Orwell’s idealized government used a complex system of "reality control". .."
http://en.wikipedia.org/wiki/Doublethink