Quincas Borba, whose full name in a Court newspaper is Joaquim Borba dos Santos, is the philosopher who invents the doctrine of Humanitas and leaves his entire fortune to Rubião. He appears in the first chapters, in Barbacena, and dies in Rio, in the house of Brás Cubas.
The book takes him up again from the Posthumous Memoirs of Brás Cubas: according to the narrator, he is "that same castaway of existence," who appears there as a beggar, an unexpected heir, and the inventor of a philosophy. On arriving in Barbacena, he falls in love with a widow, Maria da Piedade, Rubião's sister, whom a pleurisy carries off. This "little romance" links the two men, and the philosopher carries with him "that little grain of madness" which a doctor supposed he had.
Ill, he explains Humanitas through the death of his grandmother and asks Rubião whether he is his friend. He records the will, entrusts the dog to Rubião, and leaves for Rio, weeping for the dog. From there he sends a letter in which he writes: "Who am I, Rubião? I am Saint Augustine." A Court newspaper reports his death: "having endured the illness with singular philosophy," and adds: "Already then he was raving." Brás Cubas writes that he died in his house, where he had appeared some time before "in tatters and squalor."
The book does not show the death scene, which arrives by newspaper and by note. The book does not answer whether he was mad: Rubião takes the letter sometimes as delirium, sometimes as a joke. After his death, the doctrine remains in Rubião's head, who understands the allegory of the starving tribes only after inheriting.