One of Castorp's two main mentors throughout the novel, he stands for the ideals of Western civilization, the Renaissance, and Enlightenment — in short, reason, individual liberty, humanism, and progress. Though Mann's sympathies lie with him rather than with his opponent Naphta, he also shows that Settembrini fails. He fails because he embraces his ideals too fervently and loses sight of reality in the process. Typical of many libertarians, he condemns every conceivable manifestation of the metaphysical, not realizing that his own boundless idealism for the cause of humanity has metaphysical origins. He condemns the church for its symbology, hierarchial structure, and rule of obedience, ignoring the fact that it is precisely these features which distinguish the Masonic order of which he is a member. He also refuses to accept humanity's sensual component, which also has a function in the realization of his humanistic dreams. He insists on seeing humanity as pure rationality.
The futility of purely intellectual argument becomes most apparent when Mynheer Peeperkorn is introduced. He dwarfs the Italian by his mere presence.