CONFRONTED with Iris Murdoch's 20th novel, the reviewer struggles for the right words, painfully aware that Murdoch herself will have the last, best word, as in her description, cunningly concealed within the text, of her own photograph slyly gazing out from the back cover of Nuns and Soldiers: "She had increasingly the dotty smile of a Goya peasant."

Murdoch intimidates not because she is so good or so prolific a writer but because she is also a working philosopher. One feels there must be some metaphysical system that explains the adultery and musical beds, the overcharged emotional atmospheres and meticulously developed characters of the novels. It seems inconceivable that this interpreter of Plato and Sartre, apparently cloistered behind the walls of Oxford scholarship and respectability, could know so scandalously much about the lusts and passions of ordinary folk. Murdoch the philosopher writes about freedom and moral choice; Murdoch the novelist is a comic genius, a goat-footed enchantress beckoning the reader to a toot in the pubs of literature for some good gossip and a heart-to-heart talk about the eternal verities.

Like all her novels, Nuns and Soliders is about love, about what Thomas Hardy called "the fret and fever, derision and disaster that may press in the wake of the strongest passion known to humanity," the love that Murdoch calls "the real, the indubitable and authoritative Eros: that unmistakable seismic shock, that total concentration of everything into one necessary being, mysterious, uncanny, unique."

Nuns and Soldiers also has a great deal about religion, which "is to do with the destruction of personality." If Eros, then Thanatos: "It's the endings that are so terrible. That one can lose someone forever. That one has to decide. There are eternal partings . . . nothing could be more important than that. We live with death. Oh with pain yes -- but really -- with death." The religiosity is not at all as somber, or as pervasive, as these quotations may sound. The bizarre and the outrageous burst through the homilies. God does not get between the sheets, though Murdoch's admirers, whose appreciation amounts to an addiction, will want to know that Jesus Christ and two angels make an appearance, in a sensational dream sequence as unforgettable as Muriel Spark's short story, "The Seraph and the Zambesi," which similarly ventured into the supernatural.

Having cited the new novel's grand themes of love and religion, we may proceed swiftly to admire in it what Murdoch does best: the eruption of bohemian mayhem in the midst of a carefully set up bourgeois scene. Thus, as the novel opens, Guy Openshaw, scholarly civil servant, lies dying of cancer in his London flat. His distraught wife Gertrude, his friend the Count, a soulful Pole, and his relatives -- the brilliant, successful, half-Jewish Openshaws -- dutifully gather to mourn. Enter Anne Cavidge, Gertrude's best friend, who after 15 years has left a Catholic convent. Meanwhile, in the kitchen, the scruffy artist Tim Reede is secretly pillaging the refrigerator, stuffing his pockets with leftovers so that he and his mistress, the blowsy, lovable Daisy, may feast later over drinks, lots of drinks, in the Prince of Denmark pub. "Compared with sleek well-cared for Gertrude, Daisy was a shaggy ill-fed beast who wintered in the open."

Murdoch is the great chronicler of English art students, the Gulley Jimsons-to-be, the "wanderers, misfits, flotsam and jetsam, orphans of the storm, babes in the woods, mendicant artists, destitute hedonists, on a perpetual picnic." It is proposed in the Prince of Denmark that Tim marry someone rich, to support Daisy, for after all "Cash is real, cash is earnest." This is the hinge on which this compulsively readable novel turns.

"'Time, gentlemen, please,' announces the publican. 'There are ladies present!' shouted Daisy, banging her glass on the table. She shouted this every night at the Prince of Denmark. People sometimes walked along the road from the Fitzroy to hear her." Murdoch is off and running, and in no time the minuet of lovers begins: Tim and Daisy, the Count and Gertrude, Tim and Gertude, the Count and Anne. Who will end up with whom and in whose bed? Declarations of love are made and, more significantly, not made, perhaps a form of moral dishonesty.

The soldiers of the novel are from the Catholic fringes of Europe, and there are discursions into religious geography -- "Ireland is a bit like Poland after all, a miserable stupid mixed-up country betrayed, by history and never able to recover from the consequences." Its nuns take and relinquish vows, are blessed by tasks, are secret anchoresses beset by the sexual snares of the workaday world, in the face of which "what does it matter whether Jesus Christ redeemed the world or not, it doesn't matter, our minds can't grasp such things, it's all too obscure, too vague, the whole matrix shifts and we shift with it. What does anything matter except helping one or two people who are nearby, doing what's obvious."

It is an axiom of Murdoch criticism that her earlier novels are the best -- the brilliant series that began with Under the Net in 1954 and ended with The Bell in 1958. It is certainly true that these works had a freshness and ingenuity which, along with Brian Moore's The Lonely Passion of Judith Hearne and The Feast of Lupercal and Muriel Spark's Memento Mori and The Prime of Miss Jean Brodie in the same years, promised bright things from the younger British novelists. Unlike the other two writers, however, Murdoch has stayed the course. It has been a dazzling long run. At 61, she exhibits an unflagging technical mastery and seems to have entered a mature phase, sounding ever deeper currents of the human condition. With Nuns and Soldiers and her 1978 novel, The Sea, The Sea, she has produced startlingly original and thoughtful works equal to and in many ways surpassing the earlier triumphs.