This article is the account of an investigation of the meaning of a single line in the play Macbeth, a line which had been for many years a puzzle to me, until, in the course of a quite different inquiry, an answer finally suggested itself out of discoveries made in trying to reconstruct a vanished mental world which was quite familiar to Shakespeare and his audiences. My title refers to the point in the play Macbeth just after the discovery of Duncan’s murder by Macduff and the slaughter by Macbeth of the two drugged and innocent grooms. When Macduff, in that dagger-pointed atmosphere of suspicion which always follows assassinations, whether of ancient Scottish kings or of modern American presidents, asks Macbeth ‘Wherefore did you so?’, Macbeth replies: