There is a running joke throughout the episode in which the Doctor creates an apparent ontological paradox by inspiring Shakespeare to borrow phrases that the Doctor quotes from his plays. Examples of this include the Doctor telling Shakespeare that "all the world's a stage" (from "As You Like It") and "the play's the thing" (from "Hamlet"), as well as the name Sycorax from "The Tempest". However, when Shakespeare himself coins the phrase "To be, or not to be", the Doctor suggests he write it down, but Shakespeare considers it "too pretentious". In a different version of the joke, the Doctor exclaims "Once more unto the breach", and Shakespeare initially likes the phrase, before realising it is one of his own from "Henry V", which was probably written in early 1599. When questioning Shakespeare about witches, Martha remarks that he has written about witches; a reference to "Macbeth", which Shakespeare denies. At the time in which the episode is set, Shakespeare had yet to write "Macbeth" or "Hamlet", which prominently feature the paranormal, such as witches and ghosts.