Again there is a reversed double meaning to Monygham's mention of Decoud. Monygham mentions him as an idealist who led Nostromo into the service of ideals ("all this"), and says that Nostromo should have exercised skepticism on his own first. But allegorically Decoud is the skeptic, and his example in the novel has "led" Nostromo into skepticism ("all this"), so that the reversed, hidden advice here is, "You should have been sure of your ideals before venturing down the skeptical path."