One cannot overestimate the importance of Hamlet's promise "to put an antic disposition on." Not only does he promise this, but he says that he will do so "perchance hereafter." What does this mean? Quite simply, Hamlet's plan is to act crazy for the duration of the play. Why? At this point, Hamlet wants to test the different characters and family members to figure out whether the ghost's words are true. (However, could this really just be his very first delay in action?) The serious question for most scholars becomes one of appearance vs. reality: is Hamlet simply appearing crazy, or is he really crazy? If the answer is the latter (or even if it's not), the theme of mental illness is present here. It will not be the last time we see it in this play.