In part 1 of Fahrenheit 451, Montag's conversations with Clarisse trigger in him a recollection of a time when he was more emotionally alive, but he has difficulty retrieving these memories, showing how deadened he has become. In part 2, Montag's desire to remember his own past, which is still lost to him, coincides with his increasing quest for the collective memories stored in books. In this section he is growing inwardly as a person but is experiencing pain and confusion. In part 3, as he joins Granger and his followers as he leaves the city, memories begin to flood back to Montag, representing new hope for the the future and his embrace of his full humanity as the novel ends.
In part 1, the conversation with Clarisse and Millie's attempted suicide trigger in Montag a new realization of his dissatisfaction with his life despite his successful job as a fireman. He tries to recall old feelings and even how he met Mildred, thinking memory is important, but he comes up empty-handed. He is too used to living in a dulled down present. At this point, he is more or less a typical representative of his society.
In part 2, Montag still has trouble accessing his own old memories, but he is increasingly unhappy with how he is living and seeks out Faber and books as a solution. He is en route to rebellion. He says to Faber,
I've tried to remember ... But, hell, it's [what the "it" is unclear to Montag] gone when I turn my head.
Faber offers to help him remember through reading to him:
I'll read so you can remember. I go to bed only five hours a night. Nothing to do. So if you like; I'll read you to sleep nights. They say you retain knowledge even when you're sleeping, if someone whispers it in your ear.
Montag's determination to live differently and more deeply than his shallow society demands shapes and sharpens his character in the section: memory and the past are important to him, and this value drives his decisions in part 2. In this section, too, the link between personal memory and the collective memory of books is established.
In part 3, Montag escapes the city and joins Granger and his group. Now, personal memories come flooding back as Montag embraces a life of preserving the collective memories in books:
I remember. Montag clung to the earth. I remember. Chicago. Chicago, a long time ago. Millie and I. That's where we met!
Montag also remembers what he has read in the Bible. As Granger tells him,
We pick up a few more people that remember, every generation.
Preserving both personal and collective memory is shown as important to preserving the human race, and Montag comes to new life and purpose as he embraces both.