The Past the Present and the Future belong to the Party.
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The record of the past can be easily manipulated if left unchecked.
In George Orwell's novel 1984 a large political organization known only as the Party controls its populace through the manipulation of recorded history, such as rewriting documents, newspapers, figures, and even erasing photographs of political enemies. However, in the collective mind of the people a memory remains of what has been erased but the Party has found ways to repress these memories, in a act known as doublethink which is where they willingly repress these memories.
And if all others accepted the lie which the Party imposed - if all records told the same tale - then the lie passed into history and became truth. "Who controls the past," ran the Party slogan, "controls the future: who controls the present controls the past." ... Whatever was true now was true from everlasting to everlasting. ... all that was needed was an unending series of victories over your own memory. "Reality control," they called it: in Newspeak, "doublethink." (1.3.18)
This is related to the theme because due to people not standing up against the Party the Party is given free reign to alter history as they feel fit or as they need.
As soon as all the corrections which happened to be necessary in any particular number of The Times had been assembled and collated, that number would be reprinted, the original copy destroyed, and the corrected copy placed on the files in its stead. ... Day by day and almost minute by minute the past was brought up to date. in this way every prediction made by the Party could be shown by documentary evidence to have been correct... (1.4.8)
This quote relates to the theme by stating how and why the Party has gained its power over the people through the act of manipulation of the past they can control the present and vice versa.