Released as the third and final single from their sixth album Hail to the Thief in 2003. The song reached number 15 on the United Kingdom singles chart.
The song's title recalls the symbol of unreality from George Orwell's novel Nineteen Eighty-Four. In the book, inhabitants of an authoritarian future state are made to engage in doublethink, replacing their own conscience and beliefs with those imposed from above. The 'Thought Police' in the novel coerce self-aware citizens into admitting that two plus two equals five to prove the point that even though two plus two does not logically equal five, logic does not matter when no one else is willing to agree that two plus two equals anything else, under threat of pain or death.
Hail to the Thief lists subtitles, or alternative titles, for each of its songs. The alternative title for '2 + 2 = 5' is 'The Lukewarm'. Singer Thom Yorke has mentioned it as a reference to the works of Dante.