Settembrini defends reason, progress, democracy, and the rights of man; Naphta defends faith, the authority of the Church, God's rule over the earth, and terror as the means of reaching it. One wants to free the individual, the other wants to subject him to an absolute.
The opposition appears in nearly every subject. On illness, Settembrini says in chapter IV that it "is absolutely not noble," and Naphta replies in chapter VI that "the sicker, the more human." On knowledge, Settembrini trusts reason, and Naphta quotes Saint Augustine: "I believe so that I may understand." On politics, Settembrini calls democracy a corrective to every absolutism of the state; Naphta says the proletariat has taken up the work of Gregory the Great, founder of the City of God, and arrives at the "necessity of terror."
The novel grants neither of them the argument. At the end of one dispute, the narrator records "the great confusion," and Hans Castorp, in the snowstorm, calls both of them charlatans. The ending repeats the refusal: before Peeperkorn, a man with no idea at all, the two are left powerless; in the duel, Settembrini fires into the air and Naphta kills himself.