Mark Helprin has spent much of his life trying to make Ernest Hemingway appear timid. Among other things, the young author has fallen through the winter ice of the Charles River, fully clothed; sneaked into a resort hotel by substituting his boxer shorts as a tennis outfit; walked from the Catskills to Buffalo, carrying luggage; and served in two Arab-Israeli wars.
Acquaintances say he also takes great pride in getting into places unnoticed. They refuse to divulge his techniques, although one mutters something about climbing up the outside of buildings. (''I don't do that sort of thing anymore, '' Mr. Helprin says. ''It takes too much time.'')
In the past, the immaculately preppy Helprin has been much praised for the freshness, moral depth, and mature optimism of his writing.
Now he has produced perhaps the most controversial novel of the fall publishing season, ''Winter's Tale,'' in which a horse flies, people travel through time, and justice descends on New York City. Most critics either hate the book or love it.
They agree on only one thing: ''Winter's Tale'' is utterly unlike any other book currently on the best-seller lists. ''Remarkably different from what's being written today,'' says James Silberman of Summit Books, who bid unsuccessfully for rights to the novel.
Much current serious fiction in the United States has a single story line and relatively straightforward themes, says Mr. Silberman. ''Winter's Tale,'' by comparison, is an ambitious attempt at plot-juggling: It begins with a Victorian-era burglary and ends with the building of a bridge at the turn of the 21st century.
''It's historical, but not in the way most historical novels are,'' Silberman says. ''It's fantastic, but not standard fantasy.''
Helprin himself complains that many novels today feature a ''very sophisticated pessimism and an inward turning,'' with writers writing novels about writing a novel and getting divorced.
The characters in his own work, on the other hand, tend to be somewhat mythical, skilled at surmounting obstacles to triumph in the end. Wounded soldiers survive on willpower; immigrants build a new life with only their wits; a depressed man regains strength by climbing mountains in his mind.
''An optimist I am, yes. That doesn't mean I think everything's going to turn out all right. But I think ultimately everything serves a grand design which is worth seeing to the end,'' Helprin says.
Helprin, the son of a pioneer film executive and an actress, educated at Harvard and Oxford, has the grave face of someone who must often appear innocent on short notice.
In most things he delights in traveling the difficult road. After serving with Israel in the 1967 Arab-Israeli war, he worked his way from Europe to the US on a British freighter. ''I had an airplane ticket,'' he recalls, ''but I was 20 years old, and when you're 20 years old, so what? I threw it away.''
The ship's stewards were midgets whose favorite sport was terrorizing chickens, or so Helprin claims. His recollections are apparently not always to be trusted, though his father says the story about walking from the Catskills to Buffalo, carrying luggage, is true. That trip was the first leg of a summer's tour of the Northeast. Helprin says he began the journey with only $5, which he never spent and was eventually donated to a newsboy named Rudi, in Toronto.
Along with his exuberant adventurism, Helprin has set himself some decidedly unliterary career goals. A serious student of international relations (he has a graduate degree in Middle Eastern studies from Harvard), he says he loves politics, and plans, at some point, to be a candidate for elective office.
Writers should ''do something else [besides write], have a real career which teaches you lessons grown-ups learn,'' he says. ''If you want models, look at Wallace Stevens and his insurance company, Dante and Milton being diplomats, and Melville, who was a customs inspector.''
Before this year, Helprin had published two collections of stories and one novel; all were received well by critics and had modest sales.
But the publishing industry has now been gripped by what a Wall Street Journal critic calls ''quality-lit fever'' over Helprin's new book, ''Winter's Tale.''
Helprin's agent, Wendy Weil, says that when she auctioned off the book last fall, she received bids from nine publishing houses. Harcourt Brace Jovanovich won with an offer of $250,000 in cash, plus an advertising budget of $100,000 - large sums to risk on a young author whose book would probably never be made into a TV miniseries.
It is a very difficult book to summarize. It opens in the years preceding World War I, when Peter Lake, a master burglar and mechanic, attempts to rob the New York mansion of the wealthy Isaac Penn - and instead falls in love with Beverly, Isaac's beautiful daughter.
They marry. She dies. He departs on an involuntary journey through time. One hundred years later he reappears and, with younger Penns and various hangers-on, attempts to guide the city of New York through the horrible waning hours of the 20th century, into an era of justice and peace. Along the way, a horse flies. The Bayonne Marsh in New Jersey harbors warrior clam-diggers. A newspaper hires a critic of the moral performance of the city. It is almost always cold, clear winter.
''It is primarily,'' says Helprin, ''a story which is meant to be enjoyed and to be somehow uplifting.''
Among critics, the book has provoked extreme reactions. The Sunday New York Times Book Review nearly swooned with ecstasy: ''. . . the novelist commits himself throughout to the pursuit of nourishing truths - truths of justice, hope , and cheer remote from the more fashionable truths of alienation and despair,'' it said in a front-page review.
Newsweek, on the other hand, dismissed it as a ''disaster . . . a great glossy pudding of a novel'' full of ''hundreds of pages of tired and imprecise language.''
Helprin snarls a bit when Newsweek's name is mentioned, muttering that the magazine's reviewer was probably too busy reading the Hitler diaries to finish ''Winter's Tale.''
He started the novel in 1976, when he was living on a turkey farm outside Oxford (address: Mark Helprin, Lower Flat, Lower Farm, Lower Radley, Oxon., England) and studying Renaissance voyages of discovery, under Hugh Trevor-Roper.
The climate, he says, was so humid it was as if he were living under an atomizer. He thought longingly of the clear, freezing winters of New York.
It took him five years to write the first draft, in longhand, using his favorite Mont Blanc fountain pen. Another year was spent revising the manuscript on a Xerox word processor. Helprin says he does not find this mix of writing technologies to be contradictory.
''You know what I'll do if I get a big US paperback sale?'' he says, eyes dreaming behind his horn-rims. ''I'm going to upgrade my computer disk drive. Get 8-inch disks, instead of 5 1/4.''
''Winter's Tale'' has so far spent five weeks on the New York Times best-seller list. In England, the soft- and hard-cover rights have been sold for the goodly sum of (STR)70,000 (about $105,000).
''Oh, I am doing very well,'' he says. ''There may be more [money] coming, but don't forget - it's not steady. And I'm in an unusual position: Most people don't make a decent living at this.''