What is meant by the ‘mutability of the past’? Give an example from the book and explain the significance of it. 300-400 words please.

Orwell's novel is essential about power and
control--specifically, the frightening prospect of a totalitarian government controlling
every aspect of society. One of the most important means of control exerted by Big
Brother is that over what is known about one's
history.

Winton's work in The Ministry of Truth is to
"rectify" history in order to make Big Brother and IngSoc appear in a favorable light.
This is very clear in Part One, Chapter Four. One example is Winston's creation of
Comrade Ogilvy, who though fictional will "exist" due to Winston's account of him. The
Party needed a hero and Winston manufactured one. Another is Winston's realization that
while the Party is pleased to announce that the chocolate ration has increased, he knows
that it has actually been reduced.

Essentially, the power
to control information about the past allows the government to control the present. If
you are sure that something happened--perhaps even witnessed it yourself--but no one
acknowledged it, did it really happen? History is only relevant because we choose to
make it significant. Big Brother goes one step further and manufactures history. The
Party wipes clean any reference to what is not favorable and creates lies to suit its
purposes.

Goldstein attempts to explain this in "The Book"
in Part Two, Chapter Nine, but fails to articulate the full importance and significance
of the mutability of the past and the essence of "doublethink." He can explain the "how"
but not the "why."