In the end the Party would announce that two and two made five, and you would have to believe it. It was inevitable that they should make that claim sooner or later: the logic of their position demanded it. Not merely the validity of experience, but the very existence of external reality was tacitly denied by their philosophy.
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The whole point of the Party is that the people will be so trained to accept whatever the Party wants them to believe. However, by the end of the novel, the Party almost begins to believe its own hype. They have to create an external reality very different from what reality really is. They know that the statement is not true, but they cannot go back to telling the truth.
While the Party insists on people believing the truth, the truth becomes malleable. The party can make anyone believe anything. Truth is simply what one is told. Any obvious differences in what are and what the party says becomes a moot point. It is the party that is always right; two plus two made five. Next week two plus two could make six or seven hundred and fifty two, it really doesn't matter.