The main theme of The Idiot is what unscheming goodness can achieve in a world where everything has a price, and what compassion costs the one who offers it. The book does not ask whether goodness is desirable; it asks whether it survives.
Part One puts the price on stage: Totsky's seventy-five thousand, Rogozhin's eighteen, forty and a hundred thousand, the hundred thousand thrown into the fireplace; the prince is the only one who does not bid. Part Two shows compassion before adults: Rogozhin hears it as superiority, Burdovsky's group as charity, Hippolyte as an insult, and the prince arrives at the maxim that "compassion is the primary and, perhaps, the only law of the existence of all mankind." Part Three gives the objection a voice, in Hippolyte's Explanation. Part Four collects the bill.
Around this axis appear the minor themes: beauty, which the prince cannot judge and which Hippolyte feels as exclusion; faith, tested by Holbein's Dead Christ; madness, in the word idiot; death, in the scenes of the guillotine and the execution the prince recounts; and truth and lying, in General Ivolgin and Lebedev, who confess and then lie again in front of someone who takes them in.
There are two readings of the ending. In the first, goodness is defeated by the social machinery without being refuted. In the second, it sees everything and chooses no one, as Radomsky accuses. The text sustains both.