The Strange Case of Dr. Jekyll and Mr. Hyde is perhaps the purest example in English literature of the use of the double convention to represent the duality of human nature. That Dr. Jekyll represents the conventional and socially acceptable personality and Mr. Hyde the uninhibited and criminal self is the most obvious aspect of Stevenson’s story. The final chapter, which presents Jekyll’s full statement of the case, makes this theme explicit. In this chapter, Jekyll fully explains, though he does not use the Freudian terminology, that what he has achieved is a split between the id and the superego.
Until Jekyll’s letter explains all, Utterson tries to find naturalistic explanations for events that seem to deny such explanations. The tale is a pseudoscientific detective story in which Utterson plays “Seek” to Jekyll’s “Hide.” The pun on Hyde’s name reflects the paradox of his nature, for even as Utterson searches for him, he is hidden within Jekyll. Hyde is always where Jekyll is not, even as he is always, of course, where Jekyll is. What Hyde embodies in the structure of the story is his essentially hidden nature.
A central theme throughout the story, which serves to negate verbal attempts to account for and explain the mystery, is the theme of seeing. In the opening chapter, in describing the trampling of a child, Enfield says, “It sounds like nothing to hear, but it was hellish to see.” Although Hyde gives a strong impression of deformity, Enfield cannot specify the nature of the deformity.
Utterson is a “lover of the sane and customary sides of life,” but the mystery of Hyde touches his imagination. He believes that if he can only set eyes on Hyde, the mystery will roll away. Even Jekyll himself says, “My position . . . is one of those affairs that cannot be mended by talking.” The irony is that all Stevenson has to work with is words; all that Jekyll can use to account for Hyde is words. Even Jekyll’s words are hidden, however, as if within nesting Chinese boxes, in the letter within the letter that reveals all.
When Utterson comes to Jekyll’s home, he still tries to account for the mystery of Hyde in a naturalistic way, but his explanation cannot account for the enigma at the center of the story—Hyde’s ability to hide. In the letter from Lanyon, the only man allowed to see the mysterious transformation, the reader gets an idea of the structural problem of the story: how to project the psychological reality of the double in a story that attempts to be plausible and realistic rather than allegorical. Lanyon’s letter says that his soul sickened at what he saw. It is indeed the hidden that can be manifested but not described that haunts the center of this thematically simple but structurally complex tale.
Dr. Jekyll confesses to Utterson that he has for a long time been fascinated by the duality of his own nature and he believes that this is a condition that affects all men. His obsession with his own darker side gives the novel its plot but also its profound, psychological implications. Even before the climax of the story in which it is revealed that Hyde and Jekyll are the same person, the duality of their personalities creates a tension between the good, social Jekyll and Hyde who seems to revel in causing harm and mayhem, and it looks like it is Jekyll who will be overtaken somehow by Hyde.
One of the most interesting things about Jekyll’s transformation is its psychological aspect. Hyde is portrayed as an evil-looking dwarfed man with a violent temper, while Jekyll is a respected man of science, good-natured and leader of his circle of friends. Not only are these men two halves of the same person, but Jekyll describes them as polar opposites, one good and the other evil. What does it mean, then, that once Hyde exists that he slowly seems to take over, to destroy Jekyll. Is Jekyll’s theory of good and evil too neat and clean? Hyde's takeover of Jekyll seems to suggest a less clear-cut explanation, in which the human condition is not in fact double but rather one of repression and dark urges, and that once the repression of those dark urges eases or breaks it becomes impossible to put back into place, allowing the "true", dark nature of man to emerge.
Jekyll’s disorder also reflects on the other characters, and raises the question of just how upright, moral, and governed by reason they truly are. Utterson for example is introduced as a lawyerly, kind man, and seldom seems to stray from that description. But his character is so rigid and unmoving, and even impersonal, that one could imagine he too is strenuously repressing a world of darker urges.