''Because of immortal longings,'' responds his nephew, Kenneth, who at 35 is still filled with them. ''Or just hoping for a lucky break,'' having yet to encounter one himself.

Disoriented by sexual shocks and mishaps in his 15 years as a widower, Uncle Benn, a world-famous botanist specializing in lichens, has just taken a wife some 20 years younger, a beauty but more of a torment, ''the only child of rich parents, and Balzac very specifically tells you that only children born to wealth make dangerous wives,'' Kenneth notes in one of his documented indictments. He is stung by his uncle's marriage carried out without his knowledge, or nearer the point, without his permission, ''still sore be cause he cheated on me - broke the rules of our relationship.'' The exploration of this relationship, with its widening ripples of betrayal, deceit and self-deceit, of conspiracy, of sex confused with love and, inevitably, money as stakes in a bruising confidence game, is the material of Saul Bellow's brilliant new novel.

In ''More Die of Heartbreak'' we welcome back the calamitous wit of ''The Adventures of Augie March'' and ''Herzog'' among people diligently struggling to rearrange one another's lives in their efforts to rescue or simply to define their own, the human comedy implicit in Lenin's poser: Who uses whom? We hear their voices pour from the pages engulfing a plot that is comparatively simple, or would be if left to itself, a possibility this embattled narrator never entertains for a moment. He is Kenneth Trachtenberg, raised in Paris, his father long since from Valparaiso, Ind., and thoroughly Europeanized, thriving in the Rue Bonaparte as host to literary and intellectual lights including Kenneth's first mentor in Russian studies, Yermelov, on chatting terms with Malraux and Sartre, an accomplished dancer, ''Waltz, rumba, conga, tango - when he opened his arms to a woman she could feel that she had come home,'' an embrace so often rewarded that Kenneth's mother is now ministering to real human misery in Somalia. ''My father is simply fine,'' Kenneth confides under the weight of the ''phallic cross'' he still carries. ''I'm the one with the damages.'' As a specialist in Russian studies at his uncle's university in that Midwest provincial capital his father had fled, he has come there with his ''soul in the making'' to form ''a genuine, I'd say a devouring, friendship'' with Uncle Benn. IT is a youthful book. Discussing Tolstoy's difficulties with his character Nekhludov in ''Resurrection,'' the biographer Henri Troyat has observed, ''the gap between protagonist and author has widened with age. The writer has put the ideas of a solitary thinker of seventy-two into the head of a hale and hearty man of thirty-five.'' Mr. Bellow's success in closing the gap of this same disparity in age is a triumph of his enduring skill as a novelist. But then Kenneth Trachtenberg is hardly hale and hearty, by his own account quite the other way: ''a man-sized monkey wrench,'' hearing impaired, lacking ''the sort of character that requires so much height, and this discrepancy has made me a diffident person.''

Self-absorbed, self-denigrating, self-doubting and the more severely judgmental especially of his elders, he is youth in many of its more impassioned and discomfiting aspects; but the currency of his ideas and his flashes of wit in presenting them, his wide-ranging information fused into knowledge, ''Locke to Freud with stops at local stations like Bentham and Kierkegaard,'' Swedenborg, Blake, Blok and Bely, leave no question that these ideas processed for their uses by other minds become his own once he spells them out, finding no reason for existing ''unless you made your life a turning point.'' Ideas flood the book.

''I recommend to everybody [ Kenneth never hesitates to recommend ] Admiral Byrd's memoirs,'' he says at the outset. He read the book ''because Uncle Benn, who had been in the Antarctic, insisted. Commenting on people isolated in small groups during the long polar night, Byrd says that under such conditions it didn't take them long to find each other out. . . . 'There is no escape any where. You are hemmed in on every side by your inadequacies. . . .' So in the coldest cold on the face of the earth, X-rays are struck off, showing in gray and white the deformities and diseases of civilized personalities, and your own are at the center.'' THE frozen image, this ''image of ice'' deposited with Kenneth back in Paris by his teacher Yermelov as ''a small glacier demanding that we thaw it,'' persists - in Matthew Arnold at 30 finding ''his heart was three parts iced over''; in the frozen subhuman existence of life in the Soviet forced-labor camp at Kolyma; in his own academic specialty, St. Petersburg 1913 - to a point where even Uncle Benn's specialty - the Arctic lichen spending 95 percent of its existence in solid ice, perking up as soon as the sun shines, growing an inch in 20 years millenium after millenium - surfaces in Kenneth's mind as ''the little glaciers in civilized breasts,'' emerging as a kind of totem in their devouring friendship, in Benn's ''scrutiny of secret things - total absorption in their hidden design.'' A man ''wrapped in nature. The whole vegetable kingdom was his garment,'' Benn is made to order for the purposes of his new wife and her father, Dr. Layamon.

Mr. Bellow's novels have always shown a sure instinct for the confidence game, the con man, the promoter, an instinct often colored with affection not unlike Gogol's for Tchitchikov mortgaging his ''Dead Souls.''

We have Thaxter in ''Humboldt's Gift'' blithely signing Charlie Citrine up for the cultural Baedeker of Europe (''Your name will pull down an advance of two hundred and fifty thousand. We split it two ways''); the chaotic Cantabile (''You need somebody tough and practical to handle things for you. I've given this a lot of thought''); finally, in its own cockeyed way, even the gift itself, a film scenario so egregiously awful that its success is practically guaranteed. Mr. Sammler is promoted by the inept enthusiast Feffer while his nephew Wallace is out promoting everything else. Tommy Wilhelm is hustled into lard futures by the seedy Dr. Tamkin, whose byword is ''Seize the Day.'' But there is nothing blithe, inept or seedy about Dr. Layamon, his patients and pals the big-time developers, power brokers, greenmail raiders, his fortune a piece of the action here, a percentage point there. No affection either in the author's portrayal of Doctor's crude demonstrations of his total contempt for women, and the product he's promoting, his only child, Matilda, who ''can be a real bitch, but her bitchiness will be working for you, . . . great with brilliant people and she can invite them because of you, a big name in your field. The first time they'll come because of you, and afterwards because of her. It's not that you're so asocial, but a man who likes people doesn't wind up in the Antarctic.''

The unsuitable wife, as the critic Frederick R. Karl has observed, is ''a given in Bellow,'' his protagonist a man with ''an infallible instinct to marry unsuitable women . . . always poised to bring him down as soon as he shows a fault,'' and an element that remains ''constant in Bellow's fiction, after its locus: women are always the cause of man's death, be it spiritual or physical.'' Eros and Thanatos, the deep-thighed Renata of ''Humboldt's Gift'' - a big prize apple raised by her mother ''under scientific conditions, and she's hell-bent on cashing in while she's still in her prime'' - solves the conflict handily by leaving Charlie Citrine behind and marrying a wealthy mortician.

Old foes with new faces, and here Uncle Benn confronts them again taking refuge in marriage from the demon of sexuality as the husband Matilda Layamon has chosen ''to lie on her body,'' and with her father bound to see that he pays the price for the privilege: ''If you're going to share the bed of this delicious girl of high breeding and wallow in it, you'll have to find the money it takes. And it so happens that the single most valuable piece of real estate in this town was your property until five years ago, when you were screwed out of it, chum. We think you can be made whole.''

The scheme for Uncle Benn's redemption - a little blackmail, political arm-twisting, routine betrayals - is simply Doctor Layamon's smooth updating of an earlier generation's brutal venality lingering on in the octogenarian husk of Benn's own uncle, Harold Vilitzer, a.k.a. the Big Heat (said to have once cracked a man's head in a vise), an old-time pol and ward boss as crooked as they came. More recently, Vilitzer has been the architect of a family swindle involving that piece of prize real estate and provoking a failed lawsuit against him by Benn and his sister, Kenneth's mother, that chewed up the thousands they'd made seeking the millions they'd been ''screwed out of.'' With the Justice Department finally about to descend on Vilitzer and his bought judge, Doctor exhorts Benn's connivance in making his daughter rich without costing himself a penny. ''You're entitled to live in style, a rich scientist and not just a research rat,'' Doctor Layamon tells Benn.

The scene Doctor Layamon and his daughter have in mind is a vacant apartment -''bourgeois baroque'' 1910, the feel of a Venetian palace in its spread of chandeliers and a living room the size of a sheep pasture -willed to her by a wealthy aunt. The Roanoke is, in effect, the third party to the marriage, demanding a fortune to refurbish and maintain it for the scale of entertaining they envisage in guest lists proclaimimg the level of their vulgar aspirations in being graced by Dr. Henry Kissinger. ''What a girl you married, hey?''

Willful, moody, with a brilliant mind and ''more degrees than a thermometer,'' Matilda is a great beauty. But there is something disturbing about her beauty, something ''kind of two-dimensional'' about the shoulders that reminds Benn of her father. Even in sleep, a demiurge lies hidden under the skin wrapped in her silk and down comforter where ''like a sheaf of ferns . . . there were exhalations of duplicity from that delicate, straight nose.'' HAVING elsewhere cited ''useful elucidation'' among his weaknesses, Kenneth submits: ''A beautiful woman unites herself with a world-famous botanist. He may think it will serve his needs. No, all the while she has been thinking what she can do with him.'' With all of Kenneth's compassion for his uncle, there is a sting in it. Bad enough that Benn had betrayed their relationship; but in youth's ultimate indictment, by ''assisting at the degradation of Love'' Benn has betrayed himself, wandering the Layamons' penthouse dressed in custom tweeds from Doctor's tailor, that ''plant observatory, his head,'' now done by a hair stylist. Outside looms the spectre of the huge skyscraper risen on that swindled piece of real estate, drawing closer at night in a mass of lighted windows ''bigger than the Titanic, and the fiery masts like a sign to the Children of Israel'' bearing down on him sick with ''repulsive gratitude to the Layamons for letting him be one of them, choking on lies, accusing himself before God, crying out, 'What have I done! Why am I here!' ''

One turns the last pages of ''More Die of Heartbreak'' feeling that no image has been left unexplored by a mind not only at constant work but standing outside itself, mercilessly examining the workings, tracking the leading issues of our times and the composite man in an age of hybrids. The long polar night offers a sharp image for this or indeed any well-wrought novel in its claim as art, isolating people in small groups hemmed in on every side by their inadequacies where they are bound to find one another out, which is fundamentally what the task of the novel is. Here in Kenneth's diagnosis, ''What it comes down to is that men and women are determined to get out of one another (or tear out) what is simply not to be gotten by any means,'' proof in their last futile approach to old Vilitzer when Kenneth tells his Uncle Benn, ''You can love a man without loving what he did to you'' - that love, like money, is probably best kept in the family.

William Gaddis's most recent novel is Carpenter's Gothic.

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