The book gives no single cause. It shows Rubião's madness coming on gradually, amid wealth, desire for Sofia, and the dinner guests who fill his house, and only after news of the mania spreads do others speak of having him treated. Dr. Falcão suspects that Sofia has some part in it, but this is his guess.
The signs appear in sequence. Rubião dreams that he is the emperor Louis Napoleon, beside Sofia, whom the dream makes Empress Eugenie. Two busts, of Napoleon I and Napoleon III, arrive at his desk. He has a barber cut his beard to match the bust and, answering in Portuguese, thinks he is speaking French. Then he imagines himself at Compiègne: "Compiègne lay on the road to the moon." Without the beard, he begins to alternate between himself and the emperor: "They took turns; they came close to forgetting one another."
"The news of Rubião's mania spread." With the Franco-Prussian War the crises become sharper and less spaced out; he reads every news item as a victory; the dinner guests become marshals, and the poor table seems regal to him. In his mania he repeats theories of the dead philosopher, which he had not understood before.
The voices in the book diverge on the cause. Sofia explains the illness as the love he had for her. Palha speaks of rest and of hours of forgetfulness and confusion. Dr. Falcão concludes that "the two loved each other," and Dona Fernanda finds the suspicion "without foundation, absurd, implausible." The narrator does not choose between them.