What is therein lies the rub?

"Therein lies the rub" is a phrase used to indicate that a difficulty or complication is present in a situation or problem. The phrase originated from Shakespeare's play Hamlet, where the character Hamlet says, "To be, or not to be, that is the question: Whether 'tis nobler in the mind to suffer the slings and arrows of outrageous fortune, or to take arms against a sea of troubles - and, by opposing, end them? To die: to sleep no more; and by a sleep to say we end the heartache and the thousand natural shocks that flesh is heir to - 'tis a consummation devoutly to be wished. To die, to sleep - to sleep! Perchance to dream - ay, there's the rub, for in that sleep of death what dreams may come." The phrase has since become a commonly used idiom in the English language to indicate a problem or difficulty that must be addressed.