In Part Three, Beatty realizes what caused Montag to start thinking. What or who was it?
In Part Three of Fahrenheit 451, Beatty blames Clarisse for Montag's sudden interest in thinking and reading books. He admits this at the beginning of this section when Beatty, Montag, and some other firemen deploy to Montag's house. As soon as they arrive, Beatty orders Montag to burn the place to the ground. Beatty believes that Montag was "fooled" by Clarisse's "routine:"
Flowers, butterflies, leaves, sunsets, oh, hell! It's all in her file. I'll be damned. I've hit the bullseye. Look at the sick look on your face. A few grass-blades and the quarters of the moon. What trash. What good did she ever do with all that?
In other words, it is her attitude to life and her interest in nature and history which got Montag thinking. Instead of seeing that she is crazy, like so many other people in their society, Montag was enamored by her unconventional way of thinking and understanding the world. It is this, in Beatty's view, which caused Montag's downfall and he is, perhaps, correct to blame her. Looking back at the conversations between Montag and Clarisse (in Parts One and Two), we see clearly that Montag was heavily influenced by her ideas and arguments.
In Part III Montag has already visited Faber and has his little earpiece in his ear. He returns to the fire station and hands a book to Beatty. Beatty makes references to stray sheep returning to the fold. But he tells Montag,
"That made you for a while a drunkard. Read a few lines and off you go over the cliff. Bang, you're ready to blow up the world, chop off heads, knock down women and children, destroy authority. I know, I've been through it all." (pg 106)
He is blaming Montag for reading a book and entertaining new ideas in his head. He, Beatty, then goes on the quote numerous authors with differing ideas on the same subject. By the time he is finished, Montag is very confused. That is the reason Montag went to Faber in the first place. He was confused. Now Faber has to combat Beatty's argument with his own.
"But I want it to be your decision, not mine, and not the Captain's. But remember that the Captain belongs to the most dangerous enemy to truth and freedom, the solid unmoving cattle of the majority." (pg 108)