Tongue and soul in Hamlet.
In a thematic sense, the tongue is the instrument of the soul. Shakespeare is very focused on words in this play - the players of the stage, the speeches of Hamlet, Gertrude and Claudius trying to get Hamlet to speak of his pain, Hamlet trying to urge confessions from both of them. By speaking, human beings give insight into their soul.
However, Shakespeare challenges this idea by making so much of the speech contradictory and false. Throughout Hamlet we encounter a great deal of word play, Shakespeare using a vast number of multivalent terms ranging from gross puns to highly-nuanced words that evoke a host of diverse associations and images. While Hamlet can tell this difference between a "hawk and a handsaw," the play challenges the assumption that language itself can convey human experience or hold stable meaning.
In Act 3, Hamlet states "My tongue and soul in this be hypocrites" to explain the deception that must continue with his mother. While he loves his mother, Hamlet must pretend to be deranged and violent so that he can scare a confession from her. He will speak lies and pretend emotions towards his mother that are not true, but he feels this is his best chance to make her tell him the truth.