The magical Ariel arrives on the scene (after line 181), and he plays enchanting music that puts Gonzalo, Adrian, and Francisco asleep. A moment later the music also puts King Alonso asleep. Ariel, through Prospero’s command, has done this purposely so that he can see how Antonio and Sebastian will behave.
Antonio asks Sebastian if he will take advantage of the occasion, for now Sebastian has the opportunity of disposing of his brother and becoming king himself (lines 203-05). Sebastian asks Antonio if he is awake (line 205). Sebastian can see that Antonio is physically awake, but the idea of him (Sebastian) becoming king is a dream to him. He does not think it is really possible.
Antonio, however, attempts to get Sebastian to act. Antonio tells Sebastian, “Thou lets thy fortune sleep, die rather” (212). The word fortune here means destiny, and Antonio more specifically means the destiny of Sebastian becoming king. If Sebastian does not take advantage of the opportunity, his hope of becoming king will die. It will never happen. Antonio would prefer to have the far less noble Sebastian as king of Naples. Antonio knows that Sebastian would then be a powerful and ruthless ally so that the two of them together could become the dominant powers in Italy. Since Antonio would be the only witness to the murder of King Alonso, Sebastian could easily commit the crime and not be blamed for it. He could tell the people of Naples that some enemy on the island had killed Alonso. Thus, Sebastian would usurp the crown of Naples just as Antonio had usurped the title of Duke of Milan.
Sebastian then asks Antonio to tell him how to proceed: “to ebb hereditary sloth instructs me” (218-19). Sebastian is claiming that he has inherited laziness (sloth) because he was the second son born to a king. The first son inherits the crown, as Alonso had. Sebastian is metaphorically lazy in that he crawled out of the womb later (more slowly) than did his brother. But Sebastian is also suggesting that being second (or last) is part of his laziness. Sebastian is always ebbing (going backwards, making no progress) while his older brother is flowing (advancing, moving forward). Sebastian has become accustomed to accepting his position in life as being just the king’s brother and never the king himself.
Antonio, though, is quite persuasive. He convinces Sebastian that Ferdinand, the king’s son, has drowned and died. And Antonio adds that since Claribel, the king’s daughter, now reigns as Queen of Tunis, she will not have the opportunity to return to Naples and become the queen there. Thus, the next in line for the throne is Sebastian.