I.e., his idealism, specifically his idealized sense of self. To the doctor, who puts no faith in the self, Nostromo's idealized sense of self would be the ultimate affront. By extension, it is the People at large who succumb to the "confounded nonsense" of trust in the self, and, nonsense or not, it is quite true that it derives in no little part from the ideal of Liberty, i.e., the notion that the people are fit to govern themselves.