This expression is famously used in
George Orwell's
Nineteen Eighty-four.
In Nineteen Eighty-four, a good citizen is capable of practising
doublethink, (holding two mutually exclusive ideas and knowing
both to be true). An example of this kind of 'reality control' given to the key
protagonist, Winston, is that 2+2=5.
Toward the end of the book, Winston writes
2 + 2 =
in the dust of a table, in the cafe in which he is drinking. The scene is sorry and he is a crushed man. BUT, despite everything, the incomplete equation leaves us with a little hope that he has not been entirely won over by
Big Brother.
Little known but interesting fact:
In the original text, George Orwell actually wrote 2+2=5. From that, we would know that Winston loves Big Brother, and has lost all rebelliousness. Apparently, the (incredibly significant) numeral 5 fell out of the press, leaving the equation unfinished. This version has stuck.