The firemen believe, or some of them do, anyway, that what they are doing is good because they are eliminating ideas from society that would stir up trouble. Captain Beatty, the character who most believes in what they are doing, justifies the actions of the firemen in this way in a speech he gives to Montag:
We stand against the small tide of those who want to make everyone unhappy with conflicting theory and thought. We have our fingers in the dike. Hold steady. Don't let the torrent of melancholy and dreary philosophy drown our world.
By burning books, they are contributing to a peaceful and orderly society where people do not argue or fight over what Beatty believes are stupid metaphysical abstractions. In a society that has come to value passivity and apathy above almost all other virtues, the firemen serve as a wall between these attitudes and the ideas in books that might rouse the population with controversial thought.