Newspeak, doublethink, the mutability of the past.
Whenever he began to talk of the principles of Ingsoc, doublethink, the mutability of the past, and the denial of objective reality, and to use Newspeak words, she became bored and confused and said that she never paid any attention to that kind of thing.
The mutability of the past is the central tenet of Ingsoc.
There are no more uses of "mutable" in the book.
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a mutable substance
Love is the world’s infinite mutability; lies, hatred, murder even, are all knit up in it; it is the inevitable blossoming of its opposites, a magnificent rose smelling faintly of blood. —Tony Kushner, THE ILLUSION