IN THE LONG RUN - and novels are long runs - only serious people are interesting. It was Yeats, I believe, who said that the only two things that could involve an adult mind are sex and death. Final Payments is about both subjects, sometimes both at once. It is the most intelligent and convincing first novel I have read in years, one that combines the high moral seriousness of Doris Lessing and the stylistic elegance of Flannery O'Connor.

The subject of the book would seem to be too special to exert a wide appeal. A 30-year-old woman has devoted her first youth to nursing her father, who suffers a series of strokes. He is a retired teacher and a fiercely militant Catholic. The invalid and his daughter pass 11 tedious years during his illness in an untidy house in Queens. Then, one day, he dies - and Isabel is free. Free to do what? To live her own life. But what might that be? She doesn't know whether it's still customary to give one's hostess a box of cholocates. She cannot even talk about her past, since she discovers that her devotion to her father is dismissed as "sick."

Although Isabel has lost her faith years ago (a secret she concealed from her father), she is still armed with (or immured within) an exacting moral sense. It is not a petty list on injunctions learned by rote and obeyed through habit; rather, it is a heroic, courageous aliveness to real issues. Isabel becomes a sort of social worker who checks up on the care old people are receiving in private houses. Just as the heroine of Lessing's Summer Before the Dark is unable to reconcile her work for a posh international food agency with the pitiable fact of actual human hunger, in the same way Isabel finds it difficult to maintain a cool, jaunty manner while interviewing the desperate old. Worse, Isabel is incapable of flying smoothly into the hive of the ghastly singles' "scene," which is active, humming, efficient and amoral. Her responses to people - to the old, to her young lovers - are too real, too precise for comfort, either hers or theirs. Her eventual way of choosing her life, of imposing her standards on her own conduct, descends on the reader with all the black bloodiness of a brain hermorrhage. The last pages of this book disturbed me deeply, though the disturbance is welcome, one that has all the ethical grandeur and wrongheadedness behind the decision made by the Princess of Cleves or by the heroine of Gide's Straight Is the Gate.

The writing of Mary Gordon's book is careful, lucid, original. It is also stingingly physical ("That trouble touched me as if someone, quite suddenly, had lit a match beneath one of my ribs"). The religious training of the narrator works its way into even worldly little observations ("She wore a great deal of perfume, which for a woman of her age and build did not have even the redemptive appeal of a sexual gesture"; and "Eleanor shrugged as if I were a novice in a convent she had lived in for years").Isabel's own progress in the world is viewed in Catholic terms: "I could not love with God's intensity. BUt I would choose His mode: the impartial, the invulnerable, removed from loss." When she first sees the wife of the man she loves, she remarks to herself, "I have always worked very hard to believe that looks are not important; it is a part of my heritage to find any visual evidence of questionable value."

These sentences, well-formed as they are, do not come across as stately utterances. The plot is too well constructed, the tensions of the story too taut, to permit a static or irrelevant moment. The intensity is not only artistic; it is also visionary. Perhaps that's why I like this book so much. I enjoy novels in which one world is perceived from the vantage of another, in which one literary genre or tradition is grafted onto unlikely subject matter. For instance, to watch Nabokov elevate the pubescent Lolita to the status of Anna Karenina or to witness Genet render his drag queens as tragic Phedras is breathtaking. Similarly, when Henry James in The Turn of the Screw makes a ghost story serve as the vehicle for his most refined psychological analyses, or when Stendhal translates the highly colored characters of old Italian romances into contemporary and realistic terms, the effect is always brilliant. The rubbing together of two seemingly unrelated elements produces sparks, fire, warmth. I don't want to belabor my theory, but I suspect that Mary Gordon has told us so much about our depraved world precisely because she has written from the point of view of a medieval saint, as though Thomas Aquinas had been obliged to take over Rona Barrett's column.