This quote comes in Act I scene 3 and is delivered by the Second Witch to Banquo. Having addressed Macbeth with a series of paradoxical prophecies, the Witches turn to Banquo and deliver a typical set of paradoxical predictions. Of course, a paradox is something that is not literally true, and overtly it is not possible to be unhappy whilst being happy, which is what the witches are predicting for Banquo. Yet, it does foreshadow Banquo's fate and that of his descendants. Obviously, the witches identify the way in which he will be unhappy because of his death at the hands of Macbeth, but it also foreshadows the way in which Banquo will ultimately be a lot happier because of the successful escape of Fleance, and the way his descendants will take the throne of Scotland. Thus, Banquo's death by treachery is balanced against the success of his line in gaining the crown. Macbeth will die childless, and thus Banquo's descendants will outlive Macbeth's treachery. Such contradictory speech is typical of the prophecies that the witches deliver and the way that they play on the secret hopes and fears of characters.