“The universe tended towards chaos and entropy. That was basic thermodynamics. Maybe it was basic existence too.” 

Though Nora loves philosophy most, she has a well-rounded knowledge base given her love of books. Although thermodynamics is a scientific principle, Nora’s imploding life feels like chaos and entropy in action, and she wonders if the point of existence is just disorder and chaos.

“Three hours before she decided to die, her whole being ached with regret, as if the despair in her mind was somehow in her torso and limbs too. As if it had colonised every part of her.” 

Nora likens despair here to a physical disease that colonizes her body like cancer cells. At the end of the novel, Nora learns an important lesson about despair: Despair makes one feel like there is no way out, even when there is a way. Despair thrives on false conviction, and Nora once convinced herself that life was pointless.

“A person was like a city. You couldn’t let a few less desirable parts put you off the whole.” 

Nora makes a comparison between a person and a city when defending her love of Dan. There were red flags concerning Dan’s character, but Nora chose to ignore them because she saw other, more desirable parts of the “city” to like. As with a city, one tends to stay away from the bad parts and focus on the good places.