This paper intends to recognize and evaluate the concept of madness in the novel of Machado de Assis, Quincas Borbas. For that, first we analyzed the representation of madness in the protagonist of this novel, comparing with ideas about the same issue, from others thinkers, philosophers and psychiatrists from the nineteenth century. Secondly, we have tried to understand the same question, using other works of the same author, mainly, the short story "O espelho". The results of our analyses show that Machado not only uses the ideas of psychiatry of his time, but goes further, developing the idea that one of the origins of madness is in the relationship between man and society.