Capital Punishment
The story begins with several guests debating whether capital punishment is justified. The guests, intellectuals, come to the conclusion that it is a medieval and out-of-date practice that should be retired, a view that the story continues to support until its end. Indeed, one could consider the fact that the lawyer manages not only to survive his imprisonment, but also to educate himself through it, a thesis defending the value of imprisonment over the value of capital punishment.
Lifelong imprisonment kills slowly
While at his party, the banker makes the case that lifelong imprisonment kills slowly, while capital punishment kills quickly and thus is more humane. It is worth considering whether and in what ways the lawyer "dies" over the course of his own imprisonment: does his survival refute the banker's argument, or has he in fact lost parts of himself during his time in isolation?
Life imprisonment is better than death
The young lawyer argues with the banker that life imprisonment is a better option than capital punishment, since any life is better than none at all. This is what motivates him to take the banker's bet. The story invites us to consider whether the lawyer is right when he claims that any life is better than none at all. No doubt, the lawyer survives his time in solitude, but when he emerges he no longer desires to interact with society. Can a life without the urge to interact with others truly be a valuable life? Can we even imagine what such a life would truly consist in?
Books as a means of coping
During his imprisonment, the prisoner uses books as a means of coping without human interaction. He finds solace in knowledge, and, more generally, in learning. The story can thus be read as something of a self-aware work of literature: it contemplates the value and meaning of books, showing that "interacting" with the worlds of books could potentially be a viable substitute for human interaction.
The value of money
Both the banker and the lawyer value years of life in terms of money. The lawyer dreams of money as a paradise and gives up 15 years for it. In an unexpected twist, it is the very task he undertook in the hopes of money (i.e. living in solitude) that leads him to ultimately renounce material possessions.
The hubris of gambling
According to the narrator, the bet represents a frivolous undertaking. It is struck by the banker and the lawyer when they were both rich, in money and youth. The banker's own downfall, too, ultimately comes from him gambling his money away. Thus the story can be read as a commentary on the foolishness of gambling: it is an act derived from an obsession with material possessions, which the lawyer is ultimately able to overcome during his time in solitude.
Solitude breeds contempt
The lawyer writes one last letter when he is about to escape, explaining his motivation. He believes that he has grown wiser than everyone, and he hates them all, their motivations, reason etc. He believes that they have everything backwards and he can't understand them. So ironically, though we might expect someone released from solitary confinement to be eager for interaction with other people, the lawyer's experience in solitude actually made him desire to stay away from other, baser people.
