In the excerpt, he talks about the "two natures" and then goes on at the end to talk about how he might want to separate the two elements. Duality means the two different sides. This best fits the passage. He doesn't refer to appearances at all. He also does not speak of science versus nature, but rather how his scientific discoveries help him understand nature. He also does not intend any deception or disguise by exploring the two elements of human nature.
