In preparation for writing my first play, "Shakespeare Rising, I did quite a bit of  reading  about the play Hamlet. I wish I had taken better notes!   I've read some things that struck me as
 brilliant and would like to credit them.  One particular observation, which I wish I could footnote for you, jumped at me out of a book I was reading. And if I can ever find the citation, I will put it into a new post, called "Who's There, Part II."   (Or, perhaps someone who knows where the idea originated would kindly write me a comment.)

Here is the idea:  The first words of the play are: "Who's there?" spoken by Barnardo, one of the sentinels guarding the battlements of Elsinore Castle.  This phrase, though a seemingly ordinary one, the attempt of one guard to determine who is approaching him in this dark and isolated spot, defines the core of the play in three or more ways.  On a simple level, who is this ghost that has appeared?  Is it the ghost of King Hamlet who is dead, buried, and thoroughly replaced, or is it an evil spirit who seeks to deceive young Hamlet, to provoke an unholy murder that will damn the young man? The second level of meaning underscores the extremely personal  issue of appearance and reality: what is the truth behind the façades of Gertrude, or Ophelia,  or Claudius himself? Or all who live at Elsinore? And, thirdly, the ultimate of "Who's there" is aimed at the very core of young Hamlet himself.  How can he say to himself, or to the girl he loves, who he really is, when he has lost hold of all the landmarks that have identified him to himself?  He needs to redefine himself for himself before he can take effective action, and, in the meanwhile, in his pretense of madness, he flirts with the real thing.  Ultimately, it is with a kind of euphoric relief that he announces, at Ophelia's grave site, "'This is I, Hamlet the Dane!"