What is big brother?
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big brother is a collective name for their government, they say big brother is watching they mean you are always being watched by the government and anyone against the government shall be put through things like room 101 which contained the persons deepest fears.
The book 1984 was written at a time when America was always in fear of the Soviets and the control of communism. Big Brother in the book is the agency that is responsible for controlling the people through whatever means they feel necessary. The idea behind it is that they are always watching and knowing.
The goal of Big Brother is to keep people the same. The people are assigned their jobs, not allowed to keep any personal diaries or information, and must be visible to the television screen at all times. Everywhere they go the agency is watching them. In the event that a person does not conform they are brought in and interrogated and modified so they function in cohesion once again.
Big brother controls everything. The people see war as a constant around them and hear the messages that are transmitted to them to essentially brainwash them.