A look at a thematic statement in 1984 by George Orwell
The party can watch a persons every movement which in turn can help them control a persons every movement. Winston has feelings of resentment towards the telescreens, just as everyone should. He doesn't express his feelings, just represses them through fear.
"From where Winston stood it was just possible to read, picked out on its white face in elegant lettering, the three slogans of the party:
WAR IS PEACE
FREEDOM IS SLAVERY
IGNORANCE IS STRENGTH" (1.1.7)
The party controls what can and cannot be put up on the walls and placed as adds throughout the city. By putting their own personal slogans and sayings up, they make sure that those specific word are the only words known. If they are the only words known, they are the only words believed. With this, the party controls what people believe.