Analysis has focused on the characters' resemblance, demonstrated in shots of overlapping faces in which one face is visible and part of another is seen behind it, suggesting the possibility that the characters are one, and their duality. Critic John Simon commented, "This duality can be embodied in two persons, as it is here, but it has a distinct relevance to the contradictory aspects of a single person". If they are one person, the questions exist of whether Alma is fantasizing about the actress she admires, Elisabet is examining her psyche, or the boy is trying to understand his mother. Susan Sontag suggested that "Persona" is a series of variations on the theme of "doubling". According to Sontag, the film's subject is "violence of the spirit". Professor Irving Singer, examining the shot in which Alma and Elisabet's faces are combined, compared its repulsive effect to that of seeing Robert Louis Stevenson's character Mr. Hyde instead of his benign alter ego, Dr. Jekyll. Singer wrote that Bergman expanded on Stevenson's exploration of duality, the "good and evil, light and dark aspects of our nature", depicting it as "oneness" in the shot.