What doesn't Montag think about when talking to Clarisse in the first chapter of Fahrenheit 451?
Your question refers to the way in which, during his conversation with Clarisse, Montag doesn't really think about what Clarisse says to him or asks him, but just sticks to his own impression of the world and what he thinks he knows to be true. This is something that Clarisse with typical boldness addresses head on when she questions Montag's assertion that firemen used to actually put out fires rather than start them:
Strange. I heard once that a long time ago houses used to burn by accident and they needed firement to stop the flames.
Montag laughs in response to this, and Clarisse is quick to ask him why it is that he is laughing. This is an incongruity that Clarisse is again quick to point out:
You laugh when I haven't been funny and you answer right off. You never stop to think what I've asked you.
Therefore we can see that Clarisse is very aware that Montag doesn't think about the kind of questions and issues that she is raising in their conversation. He is so trapped in his own view of the world and what is "true" and what is "false" that he cannot accept or even think about alternative views of reality.