Lanyon plays only a minor role in the novel’s plot, but his thematic significance extends beyond his brief appearances. When we first encounter him, he speaks dismissively of Jekyll’s experiments, referring to them as “unscientific balderdash.” His scientific skepticism renders him, to an even greater extent than Utterson, an embodiment of rationalism and a proponent of materialist explanations. As such, he functions as a kind of foil for Jekyll. Both men are doctors, well respected and successful, but they have chosen divergent paths. From Lanyon’s early remarks, we learn that Jekyll shared some of his research with Lanyon, and one may even imagine that they were partners at one point. But Lanyon chooses to engage in rational, materialist science, while Jekyll prefers to pursue what might be called mystical or metaphysical science.
It is appropriate, then, that Lanyon is the first person to see Jekyll enact his transformations—the great advocate of material causes is witness to undeniable proof of a metaphysical, physically impossible phenomenon. Having spent his life as a rationalist and a skeptic, Lanyon cannot deal with the world that Jekyll’s experiments have revealed. Deep within himself, Lanyon prefers to die rather than go on living in a universe that, from his point of view, has been turned upside down. After his cataclysmic experience, Lanyon, who has spent his life pursuing knowledge, explicitly rejects the latest knowledge he has gained. “I sometimes think if we knew all,” he tells Utterson, “we should be more glad to get away.” With these words, Lanyon departs from the novel, his uncompromising rationalism ceding to the inexplicable reality of Jekyll.