Quincas Borba comes out of the Posthumous Memoirs of Brás Cubas, but it tells a new story: that of Rubião. The philosopher Quincas Borba was already a character in the Memoirs, and Machado refuses, in the prologue, to make a trilogy.
In chapter IV, the narrator addresses the reader: "This Quincas Borba, if perhaps you did me the favor of reading the Posthumous Memoirs of Brás Cubas, is that same castaway of existence who appears there as a beggar, an unexpected heir, and the inventor of a philosophy." The character is introduced again, in a few lines, for the reader who has not read the other book.
Brás Cubas himself is cited in two scenes. In the letter in which he says he is Saint Augustine, the philosopher recalls that he already stole "a watch from my friend Brás Cubas." And it is Brás Cubas who informs Rubião of the death, in a note: "My poor friend Quincas Borba died yesterday in my house, where he appeared some time ago in tatters and squalor."
In the prologue, Machado relates that a friend insists he make a trilogy with the Memoirs, with Sofia left for the third part, and he answers no: "Sofia is all here. To continue her would be to repeat her."