FOR 50 YEARS NOW Graham Greene has been making moral parables out of the public melodramas of our time. You might say that he was lucky to be born into a century that has been rich in such melodramas: world wars and civil wars, the rise and fall of dictators, the struggles between East and West and between Left and Right - all those complex and violent conflicts in which each side has seen the issues of Good and Evil as obvious. But it wasn't just luck: Greene's great gift was that he saw from the start how to deal with this material, how to make the violent history of the time reveal its moral meaning.

He did it by appropriating the conventions of the popular forms of fiction that the time has produced - the thriller, the crime novel, the spy story - forms in which the Big Battalions of society pursue and punish a solitary man who has broken their rules and disturbed their order.In the popular versions of this story - the sort of thrillers that we buy to read on airplanes or in bed at night - the pattern is simple: Society is Good, and the hunted man in Evil; and when he is caught and punished, order is re-established. So Evil was only a temporary disturbance after all, and we can sigh with satisfaction and turn out the light.

But in Greene's thrillers the morality is more complex. The force that pursues - society, the state, the System as we have come to call it - is necessary (what would the world be without policemen?), but it is also repressive, dehumanizing, and morally stupid. And the men who are pursued, though they are guilty, understand their own guilt more profoundly than the Big Battalions ever can, and so are morally superior to their pursuers.

Greene's guilty men are often repulsive (who would want to be a Greene character, any character"), but we are nevertheless drawn into their lives, made accomplices against our will. All the decent, orderly values that we live by are on the other side, but we are on this side, with the criminals and the betrayers. The melodramatic pattern of the popular thrillers has become the expression of a profound moral problem: society must punish guilt, but all men are guilty; guilt is the bond that makes humanity human.

In some of his earlier novels Greene allowed the possibility of another order - God's order - and so burdened himself forever with the label of Catholic novelist. There aren't really very many of his novels that one could accurately call "Catholic novels," and to my mind they are not, with the exception of The Power and the Glory, his best; but they have encouraged his critics to sniff in every corner for the smell of incense, and to read all his books as though they were Lives of the Saints. In fact, the novels of the past 20 years have been far more political than religious. But even at his most Catholic, Greene never suggested that religious belief could have any effect on the course of history in our time in historical terms, it is the policeman, not the priest, who wins the power and the glory.

Over the years Greene has shown an extraordinary talent for locating - even sometimes anticipating - the places where the continuing world melodrams was to be found: in the '30s he was in Mexico, in the '40s in Africa; he moved on to South-East Asia in the '50s, then to the Caribbean, and last time, in The Honorary Consul, to South America. The topicality and the exotic locales have no doubt helped to sell his novels, but Greene's point has never been that place makes much difference in human behavior: man's capacity to betray in infinite, but his capacity to invent new ways of betraying is very limited, and is not significantly extended by a change of scene. Judas in Vietnam is much like Judas in Brighton.

In The Human Factor, Greene returns to England, and to the genre of the spy novel. Maurice Castle is a middle-level Secret Service official at the London end of the British espionage system in Africa. His wife, Sarah, is a black African; she is alive and with him in England because a black communist helped her to escape from South Africa. What debt does Castle owe for her life? And how is he to pay it? What are the political limits of gratitude? On the other side are the senior officers of the Service. They suspect a leak in their African section. What are they justified in doing to stop it?What limits does humanity place on their methods?

The presence of South Africans, both black and white, and of reference to apartheid and to American and British support of the present South African regime, give the novel a topical surface, but the themes are the recurrent Greene themes: loyalty and betrayal; the conflict between the System and personal morality; responsibility and guilt; the expenses of love. For these themes, the Secret Service provides an excellent instrument: it is a System that demands absolute loyalty of its members, but secrecy and betrayal are the rules of the game they play - and that the System plays with them. The men who remain loyal to the Service do so for various reasons, none of them moral: because they are members of the Establishment that the Service protects, because they enjoy the game, because they have begun and feel they must play to the end, or simply because they have nothing else to be loyal to. The man who betrays the system does so for personal reasons, but at the end neither the morality nor the consequences of his action are certain.

There is much in this situation that is familiar Greene stuff; but there are also some interesting differences in The Human Factor. Most striking is the oppressive sense the novel gives of the helplessness of individual characters: the real players of this game are not the agents and counteragents, but the vast ideological forces that control them. No one acts freely, everyone is acted upon (the one voluntary act in the novel is the killing of the wrong man). The essential conflict, which in earlier Greene novels was usually an individual versus the System, is here System versus System - the communist world versus its opponents. Neither ideology is granted moral authority; ideology is always inhuman and amoral. The point is that in the struggle between ideologies that is modern history, life and love - the human factor - are not values, but only problems.

Religion, the one ideology that had carried authority for Greene, here appears only once, in the person of a priest Castle, desperate, enters a confessional and appeals for aid. "I think what you need is a doctor," the priest replies, and slams his shutter closed. Later, near the end of the novel, Castle thinks of his commitment to espionage as a kind of priesthood: "Like a Trappist he had chosen the profession of silence, and now he recognized too late that it had been a mistaken vocation." But there is no other vocation for him, and at the end he is as lost and useless as a de-frocked priest.

In a review of a thriller one can't say much about the ending but I won't be revealing the plot if I remark that the end of The Human Factor is more open and unresolved than any previous Greene novel has been. Of course the contest of ideologies can't be resolved in a novel; but the human factor itself is left drifting. God is not here, neither country nor people has any meaning, and love is only a helpless and lonely need.

It's hard to see where Greene can go from here: in such a world, what is a moral novelist to make his novels out of? We will just have to wait and see.