One of the principal themes of this incredible play to my mind is that of sanity and insanity and how, through Dysart's sessions with Alan, the play profoundly questions the extent to which such terms as "sanity" and "insanity" are actually social constructions.
As the play progresses we see how Dysart is trying to "heal" a boy who has committed an act that, in society's eyes, makes him insane. Dysart believes that this boy's pain can be alleviated through treatment with him. Dysart shows himself to be very successful in this sense: he is able to uncover memories that point towards an eventual healing of Alan. However, what is fascinating about this play is the way that through treating Alan, Dysart begins to lose faith in the categories of sane and insane and sees them more as social constructions that are contextualised and not permanent. He understands how what any given culture at a given time regards as "sane" and "insane" varies dramatically. At the end, Dysart is left with the haunting possibility that by "healing" Alan, and making him "sane," he might actually remove from Alan a passion and a zest for life that the majority of people are never fully able to experience, and which Dysart himself admits that he is jealous of. Note the way that Dysart talks about the kind of boring, empty life that Alan might live after having been "cured" by the end of the play.
Thus it is that this play profoundly questions our understanding of terms such as "sanity" and "insanity" and how we apply them.