In almost all of his writings , he emphasized the moral purpose of literature and in his 1858 play , Le fils naturel ( The Illegitimate Son ) , he wrote about the belief that if a man fathers an illegitimate child , then he has an obligation to legitimize the child and marry the woman . In almost all of his writings , he emphasized the moral purpose of literature and in his play The Illegitimate Son ( 1858 ) he espoused the belief that if a man fathers an illegitimate child then he has an obligation to legitimize the child and marry the woman .