Quincas Borba ends with Rubião's death in Barbacena, the city he had left, mad and convinced that he is the emperor, followed by the death of the dog that accompanied him; the narrator closes the book by telling the reader to weep or laugh.
Palha and Dr. Falcão commit Rubião to a sanatorium. Palha liquidates his assets and recovers three contos and two hundred, and Sofia inaugurates the Botafogo salons with a ball, wearing a necklace that had been one of Rubião's first gifts. Rubião disappears from the sanatorium and goes with the dog to Barbacena. Days earlier, Palha had found him sane, given him one hundred milreis, and asked the director not to hasten the discharge.
On arrival, Rubião shouts "To the victor, the potatoes!" in a street where there is almost no one. A storm breaks, the two spend the night in the open air, and in the morning he repeats the motto. Comadre Angélica takes him in, and, feverish, he says he has captured the king of Prussia. A few days later he dies, placing on his head a crown that only he sees, and his last words are "Keep my crown" and "To the victor...," left unfinished.
The dog falls ill, flees in search of his master, and is found dead in the street. The narrator then says that the reader may ask whether it is he, the dog, or "his deceased namesake," the philosopher, who gives the book its title, and ends without answering: "Come! Weep for the two recent dead, if you have tears. If you have only laughter, then laugh! It is the same thing."