Doublethink is the ability to accept two contradictory beliefs at the same time and to believe they are both true. In 1984, the Party uses doublethink as propaganda and psychological manipulation of its leadership and the public. The results are people (or in the novel a society) who lose the ability to form independent thoughts, remaining deceiving peace, because the person is completely unaware of any conflict or contradiction.
Eventually, it becomes possible for the Party to convince the public of anything, even if it’s the exact opposite of what the public already knows to be true, as the Party is able to control your memories, to choose to forget something, as well as to forget about the forgetting process.
Orwell defines doublethink as, ”To know and to not know, to be conscious of complete truthfulness while telling carefully constructed lies, to hold simultaneously two opinions which cancelled out, knowing them to be contradictory and believing in both of them, to use logic against logic, to repudiate morality while laying claim to it, to believe that democracy is impossible and that the Party was the guardian of democracy. To forget, whatever it was necessary to forget, then to draw it back into memory again at the moment when it was needed, and then promptly to forget it again, and above all, to apply the same process to the process itself.”