The mansion
The mansion is a symbol for the decrepit effects of time. The mansion requires sacrifices of their money, which they don't have, making it a white elephant. They enjoy their large home, but it kills them slowly because they can't afford it. The doctor slowly realizes this. Also, the mansion is an ironic symbol for the way money makes people lonely. They don't have to be in close community, because they can live so far away from each other while still being in the same house.
PTSD and trauma
All the characters in this novel suffer from stress related to their past traumas. For instance, they are all damaged by Susan's suicide, but also Caroline and Roderick have their own pasts with traumas. Dr. Faraday suffers the lost of his fiancee to hopelessness. The idea is clearly that without addressing PTSD, a family can slip toward deep depression and frustration.
Suicide and hopelessness
There is a symbolic moment in the book when Susan kills herself. She hangs herself for her family to find, which is an antithetical symbol for family, because instead of banding together to survive, Susan withdrew herself to die alone, in private. The suicide is a portrait of hopelessness, and the effect is that she spreads her hopelessness to her family. This is a sad symbol, and anyone struggling to understand suicide and hopelessness should know that there is hope, although this novel doesn't focus on hopefulness or success. Rather, it shows the devastation that is caused by suicide.
Marriage and family
When Caroline's story ends up in marriage, that is a symbol for the future, because new generations of people are implied. The symbol is of family, but in the end, when Caroline dies, the symbol is reversed. Instead of the effects of family being positive, Caroline's marriage symbolizes her desperate attempt to solve her problems by doing the "normal" thing. Each person who succumbs to hopelessness makes the lives of the next person more hopeless, because Susan's suicide makes Caroline depressed.
Time through motif
The passage of time is suggested by many details in the novel, like Roderick's experience of time in warfare, when he did not know whether he would survive or not (meaning he wondered how much time he might have left). When Susan kills herself, she removes herself from the passage of time, which would have killed her eventually, like it slowly kills the mansion itself. Time is the antagonist of this novel, because all the characters kick against the goads of time.
