What if all the pieces of belief and memory that comprise who I am [...] are nothing but a tragic misfiring in that gray matter between my ears? (5.103)
Jason's first assumption when he wakes up is that some sort of crazy brain problem is causing his identity crisis. Maybe he is a famous scientist and only imagined that he had a wife and family. Amnesia is a thing, after all. Yikes. We have the heebie-jeebies just thinking about it.
Will I keep fighting to be the man I think I am? (5.103)
Jason considers accepting the identity that everyone keeps pushing on him, but he just can't do it. He can't shake the feeling that he belongs somewhere else. Of course, he's right, so it's a good thing he listens to his intuition in this case.
[E]ven though she isn't Daniela as I know her, I cannot hide from how madly I love this woman, even here and now, sitting in this bed, in this wrong world. (5.460)
It freaks Jason out to meet parallel-universe Daniela, not merely because she's a successful artist in this reality, but also because his feelings for her are as strong as ever. Does it count as cheating if it's with a version of your spouse from another universe? We're asking for a friend.
"In one of those ponds out there, do you think there's another version of you that stuck with the research? [...] And there's maybe a version of me that's a famous artist?" (6.65-67)
The answer to this question is a resounding yes. You know, we usually think about our sense of "self" in terms of the choices we make, but this really throws a wrench into things. Although you may have made a specific choice, there's another version of yourself out there who went in the complete opposite direction.
I feel ashamed, like I lost a race to a better opponent. A man of epic vision built this box.
A smarter, better me. (7.168-169)
Interestingly, Jason feels jealous that there's another version of him out there who's like the greatest scientist of all time. It highlights the misgivings he still struggles with over the unceremonious way he ended his research career.
The other Jason, the one who built the box—he did this to me. (7.527)
Boom. Talk about a big reveal. Jason is now forced to confront the fact that the monster who kidnapped him and stole his life was himself. Freaky.
It's just that he's...different.
Different in a thousand tiny ways that might mean nothing and might mean everything. (9.35-36)
Daniela obviously doesn't realize that her husband has been replaced by an imposter from a parallel universe (it would be impressive if she did figure that out on her own), but she can tell that something's different. Even though Jason and Jason2 are supposedly the same person, they have very different identities.
His hands are trembling and his fingers curled in tightly, like they've been clawing desperately at something.
Only when he's ten feet away do I realize this man is me. (10.121-122)
Oh, heck no. Jason has already dealt with parallel universes, interdimensional travel, and a doppelgänger stealing his life, but this one takes the cake. If we're guessing, we'd say that this version of Jason is one that ended up inside the box without any ampoules left, leaving him forever trapped in Schrödinger's endless hallway.
If you strip away all the trappings of personality and lifestyle, what are the core components that make me me? (11.35)
This is a question everyone asks him- or herself at some point or another, but it becomes a lot more relevant when you're literally dealing with parallel-universe versions of yourself—most of whom want to kill you. Sort of raises the stakes.
It all points to the fact that my identity isn't binary.
It's multifaceted. (11.216-217)
This is the big truth that Jason realizes by the end of the novel. His identity isn't a zero-sum proposition between being a family man or famous scientist—there is a whole range of aspects to him that lie somewhere in between.