Consider that many of the events in the traditional story of Oedipus (killing his father, solving the riddle of the Sphinx, marrying his mother, etc.) have already occured when the play opens. Why?
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The answer lies in considering the purpose of Greek drama. They weren't interested (in this time) in seeing realistic new stories, but rather exploring themselves through their myths. This was the purpose of myths in general. By telling a known story, Sophocles uses dramatic irony (the audience knows Oedipus is doomed) to create the tragic effect so praised by Aristotle. Because we know the fate, every decision he makes is meant to horrify us ... we see it happening before our eyes and are forced to consider what elements damn him. The fates, his own character, or is there a difference? Only by watching the slow doom coming (rather than guessing 'what'll happen?') can we feel the true catharsis (release) when the tragic end comes.