Faith in a Metaphor : KNOWLEDGE OF ANGELS, <i> By Jill Paton Walsh (Houghton Mifflin: $21.95; 268 pp.)</i>

Except that he comes from an imaginary realm, Palinor’s waterlogged arrival on the imaginary Mediterranean island of Grandinsula could be one of the Englishman Gulliver’s travels to a contrary reality. Palinor is a prince in a land ruled by the dawning Enlightenment--a tolerant, civic, scientific society that works to the principles of empirical reason. Grandinsula is a medieval Catholic state ruled by a conscientious and benevolent cardinal-prince and working to the principles of Scholastic theology.

“Knowledge of Angels” is several different kinds of novel that come together astonishingly well. It is a philosophical argument in a line that stretches from Swift to C. S. Lewis’ “The Screwtape Letters.” Its disputation about the respective claims of scientific and religious thought, conducted three ways among Palinor, Cardinal Severo and Beneditx, a sweetly unworldly Thomist theologian, is agile and diverting.

The argument takes place not in a vacuum, but among three cultivated figures who come to respect and love one another, and thereby confront a tragedy. On the outcome hinges the question of whether Severo will be obliged to hand over Palinor to the Inquisition for burning as a heretic. It is a Plato’s Symposium in which real food--savory and fragrant--is served, and where there will be a real bill to pay.

But Jill Paton Walsh, an English writer known mainly for children’s books, has placed her discourse and drama in a special framework. It is as if she had set the text down on parchment and illuminated it. As a medieval courtly or biblical manuscript displayed trees, rabbits, hilltop villages, swooping angels, knightly processions and peasants reaping or making love, “Knowledge of Angels”--by means of writing alone--is a series of brilliant illuminations. They do not outshine the text so much as place it in a more spacious human context. As we argue and expound and narrate, the seasons pass, harvests come and go, and two pairs of legs protrude entwined from under a hawthorn bush.

Walsh makes several different uses of angels--in this intricately but elegantly constructed book each character and emblem reappears in shifting guises--but she starts off with an aerial view of Grandinsula. It is an angel’s view, and also a reader’s. The two have in common their ability to see everything in an eternal present, and simultaneously. Hamlet moves through time and dies, but to the reader he is always moving and always dying.

The island, somewhat resembling a 15th-Century Majorca, has mountains, plains, villages on each hilltop, a church in each village, little ports and a capital city. Everything is compressed and foreshortened--again, as in an illumination--so that an inch or two is a long journey. When Beneditx is summoned from his monastery to the nearby capital, the monks come out to say farewell as though he were off to China.

The book’s different stories move and turn simultaneously, united by the reader’s angelic vantage point. On the mountain, shepherds and ice harvesters--they stamp down the snow, pack it in straw and sell it in the city--capture a feral girl who has been raised by wolves. In the capital, Severo, austere and industrious, disposes of mountains of paperwork. By the seashore, fishermen rescue Palinor who has fallen from his ship passing at night, miles offshore. Up in the Galilea monastery, Beneditx, whose learning attracts scholars from all over Europe, is working on his book about the knowledge of angels. He holds to the old Scholastic view that, by the unnumbered billions, they make everything happen. In relay, they speed a javelin’s flight and give it a delicate curve; each snowflake has an angel disposing its fall.

The stories converge. A shepherd comes to Severo to ask permission for the wolf-girl, who will only eat raw meat and is starving, to be baptized. A local prefect throws Palinor into a dungeon when, offered the choice of declaring himself Christian, pagan, Saracen or agnostic, he insists that he is an atheist. Lordly in manner and assured in intellect--his princely rank at home comes not from lineage but from his skill as an engineer--he insists on being heard by the cardinal-prince. Severo is charmed and fascinated, and searches for a way to spare him.

Summoning Beneditx, his old friend, the two devise a double plan. Palinor comes from a country where atheism is as acceptable as faith. If he has never believed in God, therefore, he can be tolerated just as agnostics and Jews are tolerated because of “invincible ignorance.” Beneditx objects that knowledge of God may be innate though there is no firm doctrine on the matter. The wolf-child will be used to find out.

Severo orders her to be taken to a convent, taught to speak and then questioned as to whether she has ever thought of God. The nuns are cautioned never to mention God to her. It is a scientific control--one of the book’s many inquisitive ironies, since it is Palinor’s scientific culture that Beneditx objects to and that Severo finds troublesome though alluring. The more wholehearted and clear-minded abbess, like some present-day ethicist objecting to gene transplants, finds the experiment unkind.

Meanwhile, Beneditx and Palinor retreat to a pleasant hillside palace where the former goes through his array of learned arguments to persuade the latter that God has got to exist. Before long, curious and tired of the niggling detail of governance, Severo joins them. The three have a splendid time, eating, drinking, talking and bathing in the fountain that Palinor builds in his spare time.

It is a wonderfully companionable sequence but soon Beneditx begins to droop. His beautifully elaborated 13th-Century arguments--Walsh suggests their ornamented aesthetic appeal--are of no use against Palinor’s 17th-Century reasoning. (The book is conveniently set halfway between, in an imaginary 15th Century, with Severo as a transitional figure.) The cardinal-prince reflects that he has pitted a golden mind against tempered steel; before long, Beneditx is in despair. Severo urges him to hold on through faith--for this man of action, a failure to prove God’s existence is no more troubling than a failure to prove that the sun will rise tomorrow--but Beneditx’s faith is built on argument and it has crumbled.

Beneditx’s crisis will be followed by Palinor’s. With the arrival of a deathly agent of the Inquisition and the collapse of the wolf-girl experiment, Severo finds himself torn between delivering up Palinor and desperately maneuvering to save him. The book advances to a brilliant and relentless conclusion.

It would be relentless, that is, if it did not consist of its illuminations as well as its argument. The island and its people are vividly and beautifully portrayed. The wolf-girl, who remains mysterious to the end, and the nuns who care for her are drawn with fine nuance and an imperceptibly gathering power. The stranger, cardinal and theologian are three eras of history--the early Enlightenment, the Renaissance and the Middle Ages--meeting to test their ideas, their hearts, the different sense of sky, skyline and the world’s panorama that greets their eyes each morning, and the even more different sense of what it means to act as a human being. All on an island that Walsh’s illuminated and illuminating book has set down in the weight of reality and the lightness of possibility.