Oedipus Rex is what many consider to be the perfect tragic play; Oedipus, the title character, perfectly embodies Aristotle's description of a tragic hero, and the action of the play makes it what Aristotle defined as a true tragedy.
In the closing scene of Oedipus Rex, Oedipus, who has recently learned that he has unknowingly killed his father and married his mother, blinds himself as a form of self-punishment and prepares to live the rest of his life in exile.
The Chorus closes the play with these final lines:
People of our country Thebes, behold this Oedipus,
who knew the famous riddle and was a most powerful man,
whose fortunes all the citizens watched with emulation,(1555)
how deep the sea of dire misfortune that has taken him!
Therefore, it is necessary to call no man blessed
as we await the final day, until he has reached
the limit of life and suffered nothing grievous.
This final scene is significant because it portrays Oedipus as a tragic hero and shows audiences that not even a noble man such as Oedipus is free of sin or wrongdoing. After his reversal of fortune, Oedipus accepts responsibility for his actions (killing his father and marrying and having children with his mother) by blinding himself so that he won't have to "see" his sin--his children--any more. Further, he prepares to live a life of exile since it is the punishment he vowed to impose upon the person who killed Laius (before he understood that he himself was the killer).
Essentially, the Chorus, in the last lines of the play, is observing that no humans who are living are "blessed," as even the most powerful and respected people can fall victim to fate at any time.