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The De Laceys' story illustrates both the goodness and evil of which mankind is capable more importantly, it shows the way in which each person may be capable of both good and evil. Felix's strong sense of justice leads him to aid the merchant; his love for his family draws him back to Paris, despite the fact that he knows that he will face a stiff punishment. By contrast, the merchant who is himself a victim of bigotry and hatred betrays the man who risked his life to help him. The creature thus encounters the two contrary aspects of human nature.
Of course, Shelley's representation of the Muslim merchant as lying and duplicitous is itself an example of nineteenth-century racism. By the same token, Safie's nobility of spirit is presumed to come from her Christian mother; the underlying assumption here is that Muslims, and Turks, are not capable of human kindness.