O'Brien instructs Winston about the state's true purpose and schools him in a kind of catechism on the principles of doublethink — the practice of holding two contradictory thoughts in the mind simultaneously. For his final rehabilitation, Winston is brought to Room 101, where O'Brien tells him he will be subjected to the "worst thing in the world", designed specifically around Smith's personal phobias. When confronted with this unbearable horror — which turns out to be a cage filled with wild rats — Winston's psychological resistance finally and irretrievably breaks down, and he hysterically repudiates his allegiance to Julia.