"Alas, alas! How terrible to know/ when it does not help the knower;" In some other translations the line reads "Oh Fate! How terrible it is to know/ when nothing good can come of knowing." (Treasury 37) This is an example of the Greek dramatist's complete willingness to "give away the ending" of a play to the audience, effectively killing any suspense. Tiresias, the blind man who can see the future, is telling us what will happen by the end of the play. Since the tragedy of this play is that Oedipus unknowingly killed his father and then married his own mother, Jocasta, knowing these facts would not help Oedipus in the least. The knowledge, when it comes out, leads to disaster, with Jocasta killing herself, Oedipus blinding himself, and Oedipus leaving his motherless children behind. Tiresias' statement is not only a revelation of the plot, but is a interesting commentary on morality. If a person does not know that he or she is committing a heinous crime (such as patricide or incest,) is that crime of the same magnitude as it would have been if undertaken with full knowledge? Is the revelation of that knowledge necessarily a right action if it only compounds the evil, without expiating it? The answer is not given in the play -- it is up to the reader to decide.
Source: Treasury of Theatre, University of California Press, 1951