A loaded gun
We must all be alike. Not everyone born free and equal, as the Constitution says, but everyone made equal. Each man the image of every other; then all are happy, for there are no mountains to make them cower, to judge themselves against. So! A book is a loaded gun in the house next door. Burn it. Take the shot from the weapon. Breach man's mind.
Context: Fire Captain Beatty explains to Montag why books are burned.
Not in a duel yet
Source: Part One: The Hearth and the Salamander