The plot of Oedipus Rex is praised for the terrible irony with which every step Oedipus initiates brings him closer to disaster, to uncovering his unwitting crimes. There is another process operating, however - also an ironic one. Any step taken to avoid disaster tends to precipitate it.

This process begins before the beginning, as it were, when Oedipus’s parents, Jocasta and Laius, having been warned that their son will grow up to kill his father and sleep with his mother, actually ensure that this will happen by not killing the baby but leaving him on a mountainside, so that he can be rescued and grow up in ignorance of his real parents. As a result, Oedipus does not recognise them when they meet years later, and the prophecy comes true.

The play, like all Sophocles’s tragedies, is permeated with a more or less gloomy resignation to the essential awfulness of being alive, whether that results from chance or the sadism of the gods: in Oedipus Rex, the oracles hound Oedipus from pillar to post and eventually to destruction.

In his adopted home of Corinth, a stray remark casts doubt on his parentage. His parents reassure him, but he’s not convinced and goes to Delphi to ask Apollo who his parents are. He gets no answer to this, but is told that he will one day kill his father and sleep with his mother. He decides never to return home. On the road he gets into an argument with a group of travellers and kills them all. He then arrives (by chance? hardly!) in his real home town - the one place he was determined to avoid. There he solves the riddle of the Sphinx, is made king (the previous king having recently been murdered on the road) and marries the queen, who is, of course, his mother. The oracle has driven him inescapably to his ghastly fate, and in the most sadistic manner.

Sophocles’s conclusion in the first of these two plays is really pretty grim. The Chorus point to Oedipus (who has just found out who he is and, consequently, what he’s done) and say: “No man is truly happy - none: We only dream we’re happy. Then Almost at once the dream is gone.”

This isn’t simply the sort of pessimism that springs from empathy with one’s fellow human beings, but something infinitely darker because more profound. Oedipus is being cited as a paradigm for the human condition as a whole: “I say that; this is how I know: Your lot, poor Oedipus, is one That makes mankind’s plain to me.”

The second play, Oedipus at Colonus, is famous for its more serene mood. Oedipus the self-mutilator, the polluter, is finally cleansed and, at any rate partially, healed. His passing is something supernatural, almost divine. Moreover, he is accepted in Attica. He finds a new home there after years of wandering. But don’t be fooled. The basic mood of the play is once again dark.

For one thing, Oedipus is forced to confront two enemies - Creon, his brother-in-law, and Polynices, one of his two detested sons - both of whom have come to exploit him for political purposes. Oedipus lets them both have it in no uncertain terms. (This is a play with its fair share of anger, and perhaps underlying it is the hatred Sophocles felt for his own son). Besides, although Oedipus’s suffering may have eased, it still obsesses him. “I will die” he says, “still wretched, yes, but not entirely so”.

The emphasis must still be on “wretched”. The sufferings he has endured in his life, and the confrontation he has to face during the play, lead the Chorus to reflect on the difficulty of existence once again, in much the same vein as in the first play: “Oh, the utter folly of the man Who wants to live beyond a moderate span. For life is full of incident, far less goes right than wrong And there’s no joy for those who live too long . . . Old age is there: unloved, unlovely, weak, despised, in short Beset with sufferings of every sort In which this wretched man as well as I . . .”

In the last line lies the paradigm: the Chorus point to Oedipus as a prime example of something universal - namely, human misery. “Never to have been born is best by far, And second best, I’m certain, once we are, To go back where we came from, quickly too.”

In keeping with this gloom, Oedipus’s two daughters are left at the end of the play, both refugees, without the one thing that has given life meaning: looking after their old and wretched father. All the Chorus can offer is empty consolation: “There was suffering yesterday”, and “What’s passed has passed and can’t be changed.”

I doubt if such reflections, wise though they are in their way, fool the two girls any more than they fool us. The job of tragedy is surely to go deeply into the dark side of the human condition. If comedies help us to escape from the grimmer aspects of life by laughing at them, great tragedies like the two Oedipus plays help us come to terms with them by examining them uncompromisingly. In which regard, there is no more uncompromising tragedian than Sophocles.

Previews of the National Theatre production of The Oedipus Plays, directed by Sir Peter Hall, open at the Olivier Theatre, South Bank, London SE1 tomorrow. Tickets: 0171 928 2252.