The Unbearable Lightness of Mortality : IMMORTALITY, <i> By Milan Kundera translated from the Czech by Peter Kussi (Grove Weidenfeld: $21.95; 345 pp.)</i>

Imagine an ingenious mathematical proposition, illustrated not in traditional apples and oranges, or three men differently walking X miles, but in a set of lively and delicate characters who squeeze the oranges into orangeade, who achieve the miles in pas-de-deux and pas-de-trois , in pirouettes and plies.

It is Milan Kundera’s fiction of ideas. In his best books, “The Unbearable Lightness of Being” and “The Book of Laughter and Forgetting,” the ideas are people, just as in the play “Jumpers”--Tom Stoppard has a kinship with Kundera--they are acrobats.

In “Immortality,” the people are ideas. Kundera retains his familiar grace, irony and enlivening paradox, but he has lost some of the spark that turns thoughts into flesh. Like Agnes, the soulful but abstract protagonist of this new novel, the author seems crowded by other lives, even those he creates.

Kundera has stepped right into his book, this time not only as narrator but as frequent visitor, as if he didn’t trust the characters to get things right. He has soured on characters, in fact, and his launching notion, quite splendid in itself, has a discouraging effect on what follows.

The author sits by the pool of a Paris health club. He sees a middle-aged woman wave as she leaves. It is a glorious wave, a golden wave, one of those waves that movie stars used to give.

He will use the wave, he thinks. He will bestow it upon Agnes and her sister Laura, who are to inhabit this book in the same polarity--spirit and flesh, respectively--as Tereza and Sabrina in “Unbearable Lightness.”

Does it seem odd to give one same gesture to the middle-aged swimmer, the pure-spirited Agnes, and the sensual Laura? Not at all. There are many people but only a few gestures. The gesture doesn’t reveal the essence of a woman, Kundera writes; on the contrary, “The woman reveals to me the charm of a gesture.” We do not use gestures as instruments to express ourselves. “Gestures use us as their instruments.”

And so, in “Immortality,” we have a brilliant pattern worked in fuzzy thread; a vivid dance in which the dancers are indistinct. We will admire Kundera’s choreography without ever really experiencing it.

The story is fragmentary, told in the writer’s lovely and characteristically emotional syncopation; erratically, unexpectedly and with digressions. More accurately, the story, emerging and submerging, is a kind of digression on his digressions.

Agnes is fiery but immaterial; all soul. She has longings, some of them erotic, but the eroticism is fantasy not flesh. She is a solitary; her deepest love was for her father, another solitary. She inhabits one extreme edge of a bed with her husband, Paul, who inhabits the other extreme edge. In between is an area largely unused, though they do have one daughter.

Opposite to Agnes, and consummately jealous, is her younger sister, Laura. She is earthy, passionate and inquisitive. Though she has a lover, Bernard, who is a trendy television journalist, what she most passionately wants to acquire is whatever Agnes has. In this case, though, it’s not much: Paul. Eventually, she will get him.

Paul, a fashionable Parisian intellectual, is not so much a character as an opportunity for digression. He believes in art and its eternal values; he also believes in being up-to-date. History has recently died, therefore, high art, its product, also is dead. Willy-nilly, Paul must espouse pop-cult even if this makes him, as one disgusted associate puts it (I think I hear Kundera speaking), “the ally of his own grave-diggers.”

Bernard is another digression; in this case, it is Kundera denouncing contemporary journalism-by-image. The journalist in a democracy is the only figure who is entitled to demand that people answer him. This is totalitarian but frail; if the journalist’s image falters, he disappears. One day, a man goes to Bernard’s office bearing a poster with the words: “Complete ass.” The insult is unsubstantiated, but it is catchy. New image replaces old image, and Bernard is out.

The sisters’ rivalry leads, digressively, into the theme that serves as the book’s title. Beneath Agnes’ spiritual and Laura’s fleshly passions is a common longing for immortality. Agnes pursues it by divesting; by relinquishing things, by fleeing human contact. She runs away from Paul, and when she is mortally injured in a car crash, she dies faster so that he will arrive too late and she won’t have to see him. Like a fountain, Kundera tells us, she wants to be, but not to live.

Laura’s immortality is pursued by acquiring things: lovers, a cat, furs. They will become her, express her, extend her life. When she wants to move in on somebody, she uses objects; she invades Paul and Agnes’ home by giving them a big white piano. Kundera makes no choice between the two passions; he keeps them in the same kind of wicked balance that he used in “Unbearable Lightness.”

There is a strong parallel between the two books, even to the car crash that eliminates, in each case, the more abstract of the rivals. As a detail, both books have passages that suggest the clownish, melancholy innocence of the procession of animals in “The Magic Flute.” In “Lightness,” it was an affectionate and troubled dog; in “Immortality,” it is an affectionate and troubled elevator that follows Agnes as she goes up and downstairs. Kundera himself, speaking as his own fictional character, tells his fictional sidekick, the puckish Dr. Avenarius, that he should have saved the “Unbearable Lightness” title for this new book.

Most of the digressions are witty, though Kundera’s occasional prosiness has become less occasional, and some of them go on too long. One that seems forced is a long exploration of erotic fantasy and reality by a would-be artist nicknamed Rubens. He is Agnes’ lover. Their sex is kinky, appropriately abstract and, among the book’s other abundant clevernesses, it has the effect of one encore too many.

For all of “Immortality’s” charm and intelligence, and its undercurrent of lament for passing time and the frailty of history, it falls short of its predecessors. It has the same immortal longings, but Kundera has not given us mortals to carry them; only his own voice.

In fact, the brightest and most vivid personages in the book are not the characters in the story but two historic figures who occupy the best and most protracted of the digressions. The pursuit of Goethe, the aging poet, by the young and determined Bettina von Arnim is comic, subtle and touching. At her first visit, Bettina climbs onto the poet’s lap and promptly falls asleep; rebuffed, she keeps coming back year after year. She is not after love for its own sake, but as her own version of immortality.

It is a duel of immortalities. Goethe sends her away and then allows her back, finally indulgent toward her attempt to invent the legend of a great passion. Indulgence is not only the kinder course but also the more prudent. The great man’s own immortal legend is safely secured, of course. But Bettina is uncontainable--very like Laura--and why take the chance that she might dent it?