When Montag meets Clarisse and she questions him about firemen, he begins to think. She asks him if it's true that fireman once put out fires instead of setting them. He responds automatically that houses have always been fireproof and that firemen have always started fires. Yet the question bothers him. He brings it up in the firehouse and gets the same response that he gave Clarisse. His raising the question suggests that it has been nagging him.
Later, Montag watches a woman with books die instead of leaving her books, which the firemen are burning. This also weighs heavily on him. Because of Clarisse, he has started to look at the world with empathy, from the point of view of other people. He expresses to Mildred, finally, some doubts about the destructive nature of his profession in the following quote:
"It's not just the woman that died," said Montag. "Last night I thought about all the kerosene I've used in the past ten years. And I thought about books. And for the first time I realized that a man was behind each one of the books. A man had to think them up. A man had to take a long time to put them down on paper. And I'd never even thought that thought before." He got out of bed. "It took some man a lifetime maybe to put some of his thoughts down, looking around at the world and life, and then I came along in two minutes and boom! it's all over."