How did they prove the old man’s testimony to be invalid?

The joke is, i just got 5 free points!

The main themes of the play are: fate and free will with the inevitability of oracular predictions is a theme that often occurs in greek tragedies the conflict between the individual and the state similar to that in sophocles’ “antigone” and   people’s willingness to ignore painful truths both oedipus and jocasta clutch at unlikely details in order to avoiding facing up to the inceasingly apparent truth and sight and blindness the irony that the blind seer tiresius can actually “see” more clearly than the supposedly clear-eyed oedipus, who is in reality blind to the truth about his origins and his inadvertent crimes.