YOU CAN'T BUY a new suit at Brooks Brothers. In fact, you wouldn't want to. That would be missing the point.

You came here to get away from new things - all that boutique hype and hairdresser chic, all those Johnny Carson lookalikes and traveling salesman in doubleknits - all that stuff. You can almost forget about it here, standing in the spacious aisles and the wood-paneled hush, touching the camel's hair and cottons, the silks and soft worsted flannels that lie in your hand as if grateful to be picked up. Listen to the rustle of the tweeds.

Outside, in the streets, there in a stubbornly bewildering world of social ills, sexual competition, sudden violence, sporadic doubt. In here, in the cashmere quiet, there is . . . Brooks Brothers.

"Today, a big catch-phrase is 'life style merchandising,'" says Brooks president Frank Reilly. "It's almost nauseating - everybody's throwing around life-style. And it's comical, because Brooks has been, since 1818, lifestyle merchants. It goes way beyond the slide rule and the tape measure. It's a whole philosophy, a whole way of life.

And although that way may not be for everybody, tens of thousands of men find the Brooks style so congenial that the company turns one of the highest profit margins in the industry, performs consistently and outstandingly for its parent corporation, Garfinckel's, and has expanded as if by manifest destiny to sixteen branches beyond the main store in New York, including such doubtful outposts of gentility as Cincinnati, Pittsburg and Dallas.

And Brook has done it by selling perhaps the least exciting form of human shelter since the pup tent - the No. 1 sack suit, with its unpadded, full-cut, generously draped shape - and the predictable button-down collar shirts and a variety of almost painfully sedate neckwear.

Furthermore, they have done it with increasing success over the past twenty years - two decades in which the spastic upheavals in American style could not have been more inimical to the Brooks Brothers way of life.

First came the denim and tee-shirt Sixties when many clothing houses sank in despair. "Everybody was just unkempt and dirty," Reilly says of the period. He knew it wouldn't last. "After all, people get through that stage in their lives. They start to think about getting married and settling down and doing something"

What really scared Reilly was not a wholesale revolt in American society, but something far more sinister - the leisure suit. "Everybody was going to buy one," he reflects with distaste, "because they were cheap. That was much scarier to me than the other thing, because I felt that that just couldn't last."

Moreover, Brooks has endured the recent obsession with European cut in men's wear. While television celebrities and talk-show hosts were gasping in clothes pinched in and padded out to product a sort of fat hourglass silhouette, Brooks held the line. And now that the trend is back to the natural shoulder vested look, the company is vindicated. "European suits, you know, are like a board," says Reilly, "and they're not comfortable. I don't care what you say, they're not comfortable to slouch in as most American males slouch."

General merchandise manager Jaffe has even more definite ideas. "Italians have skinny shoulders. Unless they have padding - or the French, you know, unless they have padding - they have no shoulder at all. And the English are built slope-shouldered.

"But the Americans have an athletic build. That's why they haven't been comfortable in European clothes, and that's why they have come back to us. In addition, the look is American - the strength of the world. I don't care, the pound or the dollar or the oil or whatever, the strength of the world is in America.

These are interesting notions, but the fact remains that the company thrives on the small dependable percentage of every generation that become Brooks men, and they probably aren't driven to it out of respect for the strength of America.

Some may be attracted by the prices. The suits cost from $230 to $315 (there are no decimal points in a Brooks price tag), and some poplins and corduroys can be had for little more than $100 and change. That isn't cheap, but neither is it outrageous.

Brooks executives will tell you that their customers keep coming back because they value natural materials, simple design and comfortable fit over everything else. That is undoubtedly true, and undoubtedly irrelevant.

No, when you get down to it, the real reasons for Brooks Brothers' success over the years have less to do with clothing than with the profound and changeless psychological needs of their customers and with certain unalterable aspects of American society. Safety First

There is a kind of person for whom daily life is fraught with doubts and dangers, with potential board-room faux pas and sales-meeting gaucheries which are virtually unimaginable to the rest of us. He is troubled by nightmare visions in which two vice presidents of his company meet, and one says to the other, "Young Benson seems like a bright fellow and a hard worker. But he just doesn't seem to be our type." Think, for example, of John Dean on his first day at work.

Frank Reilly understands these fears. "One thing a customer is getting at Brooks," Reilly says, "is the self-assurance that what he's wearing will be well accepted. He's getting something, to put it in a way I don't like to think of it, that's safe. He's bouhgt it from the finest company of its kind, he knows it doesn't scream out, and so it can't be in bad taste."

Safety. It's an odd word to use. After all, we aren't talking about ski bindings or air bags here - just a couple of pounds of wool to wear behind a desk.

Jaffe puts it this way: "It's hard to make a mistake here. You'd have to search pretty haredto find things that didn't go together." That's it. Even if you're dumb as a sponge - it's nearly impossible to be wrong.

When Brooks does change, they do it knowing that they are speaking for the whole conservative tradition.

When lapels went from six to an unimaginable seven inches wide, Brooks refused to budge. And when they finally decided to widen them, they did it with a restraint that makes the movement of glaciers seem hasty and ill-considered by comparison. In 1962, the Brooks standard lapel was three inches wide. In 1963, it expanded to three and eighth; by 1965 it had grown to three and a quarter, and went as far as three and a half in 1969. Since then, the dizzying push shoulderward has continued, and the new stock at Brooks this fall can be found with lapels a full four inches wide, having come to rest again after a net increase of one inch in fifteen years.

Brooks does suffer occasional pressure to take advantage of a potentially profitable trend. Even a company with gross revenues in the neighborhood of fifty million dollars a year is reluctant to pass up profits.

For example, no one doubts that Brooks could have made a lot of money selling bell-bottomed or flared trousers during the five or six years in which that vulgar fad ran its course. But they refused. they also passed up the high-heeled or high-soled shoe, the fat tie, and the tent-collar shirt. The whole look just wasn't Brooks.

"We could have sold it," Reilly admits, "In fact, we could probably sell almost anything if we put our minds to it. But think what we'd lose in return!"

(They may have lost some of that integrity already: Brooks does indulge in some of the soup-to-nuts retailing which one associates with a Neiman-Marcus or a Sears Roebuck. Those determined to wallow nose-deep in the Brooks Brothers ambience can buy their brand of soap, deodorant or shampoo, a soi-disant "lime lotion" of unspecified utility, after-shave liquids in "woodmoss," sandalwood, citrus or "346" aromas, and even Brooks Brothers' own mouthwash with the dead sheep logo right on the bottle. Add to that wristwatches in any of three syles - with the Brooks Brothers name all too visible on the dial - and such dubious chattels as "English shaving brushes of pure badger hair," Brooks' own shoe-shine kit, yard-of-ale drinking glassess and a parti-colored "sport or town umbrella," and you have to admit that they could sell "just about anything.")

Even when Brooks does take advantage of a trend, they do it in a way that vouches safe the look they had before. Although they now offer a four-inch necktie, they also continue to sell their previous three-and-a-half-inch tie: Both are available, side by side, on the display cases, and there are a dozen Ph.D's in psychology to be earned by explaining the differences between the people who buy each width. Moreover, they still offer their original two-and-a-half-inch ties - and they are selling briskly to those whose taste has refused to give an inch since the Kennedy years. (And, to your added stupefaction, the Brooksgate collection offers polyester ties which are three and seven-eighths inches wide, and wool-and-mohair ties which are three inches wide. Brooks may not go to any lengths to satisfy its clients, but they are apparently prepared to go to any widths.)

Nowhere is the essential resistance to change so notable as in the famous and widely imitated Brooks Brothers oxford cloth button-down-collar shirt. The company originated the style around the turn of the century, taking the idea from the shirts worn by English polo players - the buttons kept the collar from flapping against their faces in the heat of the game - and it seemed perfect for the look Brooks wanted to create. It has remained unchanged for twenty years. Even today, it billows out proudly in utter disdain for "tapered fit."

So there is real safety in Brooks, the kind of safety which you cannot trust to anyone else.But that isn't everybody's problem. Some customers have other needs. The Prisoner of Sex

For most American men, decisions about their clothing pass naturally from their mothers to their wives - at what terrible mental cost no one has yet reckoned. Boys are brought up in our culture to shun interest in clothing - probably because mommy wants to believe that within her son lurks a little Huck Finn whose puckish spirit is most at home going barefoot on a fishing wharf. As a result, she buys his clothes, and he never learns anything about colors or fabrics. When he does grow old enough to choose his clothes, he probably does it wrong. After that, his wife goes along.

Jaffe, who knows as much about men's shopping habits as anyone in America, understands that Brooks represents an end to all that. "We probably have less man-and-wife shopping than any department store or any other speciality store," he says proudly, aware that the man-and-wife shopping trip can be an emasculating experience:

"You go into a department store, and there's a guy and his wife, and they've trying on fifty suits. I've always had a theory about that - the wives take out all their aggressions in front of the mirror. They get in front of that mirror and they push their husbands. 'Here,' they say, 'this doesn't look good. This wrinkles. This is wrong.' And what they're really saying is, 'You look lousy.' They can't say it to their husbands' faces, but they sure can do it with the fitter, using the suit as the medium.

"Our customers don't do that. You stand around here and you see a man doesn't try on a lot of suits before he buys one, or he doesn't look at a lot of ties when buys one." And, of course, he doesn't shop with his wife.

At Brooks you buy clothing which is designed and tailored by men, and whose principal function is a form of communication to other men. A man who shops at a store like Britches, by contrast, is probably dressing for women - to get the kind of mock-English singles-bar look that seems right for the scotch-and-water scene at Clyde's. That's why it seems so natural to bring his wife or girlfriend along. But Brooks is a store for men to whom clothing is not a sensual or sexual medium. That is the traditional view of men in this society, albeit a somewhat strange one.

In the animal kingdom generally, it is the male of the species who has the most brilliant coloration, and that distribution of hue serves to attract females for breeding. Darwin observed the phenomenon in peacocks, remarking on how the male bird dances in the dust, spreading its brightly colored tail for display to the admiring pea-hens, who selectively breed with the best and the brightest. (The human equivalent of this spectacle may be observed in Georgetown on weekends.)

But somewhere in Western society, this natural process was reversed. Toward the end of the 18th century, men's clothing began to turn dark and stayed that way; and in the first part of the 19th, as what was to become the modern business suit was taking shape, darker colors were joined by the baggy jacket and full trousers which didn't so much follow the body as cover it.

The end result, on sale at Brooks, is the sack suit.

When they try to describe the look created by these suits, Brooks executives are driven to negative expressions: not flashy, not padded, not shiny, not too much decorative thread. And although the company has never been reactionary about color - they were the first to introduce the pink shirt to American acceptability, and their sportswear is hardly dull - the quality they praise most highly in the buyers they employ is the cultured eye, the unerring ability to say, "This is not Brooks." Conscientious Objector

In the moneyed ranks of American life, there is a subset of men who are intensely involved in their own clothing, whose interest has passed over the narrow line between considerateness and vanity. With zeal just short of narcissism, they spend a large part of each day considering their dress, doting on it, adjusting it.

They are not Brooks. In fact, by choosing to shop at Brooks Brothers, a man is choosing to avoid the clothing game, not so much to withdraw from the competition as to refuse to enter it. There is a humility to a Brooks customer that goes well beyond the Protestant mind-set that revolts at too much glorification of the flesh. There is a fundamental belief that, while it is necessary - and even a duty - to dress appropriately, in a manner that people can trust, it is simply demeaning to spend too much time on it.

Reilly, in his quiet pinstripe, says: "Our customers, do not want to use their clothing as an expression of themselves. They are not people who want to walk into a room and be noticed for a suit. They want to be noticed for what they are - clothing is not the single most important thing in their lives."

There has always been a market for the look and style of Brooks Brothers, and there always will be. In fact, now that the trend in fashion is away from the European cut, Brooks will probably be abnormally popular for a while. Jaffe sees a bright future in the near-term: "A few years ago, everything went far away from traditional-type clothing, and it's now coming back. It'll be harder to recognize our suit - unless you're an expert - next year."

But Brooks will obviously never be stylish in the ordinary sense of the word. That would be missing the point. Whatever happens, whatever the next fad is - shoulder fins, pogo-stick shoes, tablecloth ties - Brooks isn't going to do it. Not ever.

To Frank Reilly, the Brooks look is a moral trust maintained from one decade to the next, because generation after generation has trusted the company with an unquestioning faith that they give to virtually no other institution in modern life. When Brooks speaks excathedra through Reilly, it does so with a gravity that goes far beyond mere suits on racks or sales revenues. "We are unquestionably the Establishment," he says simply, "and we have enough customers who believe in us that we will never do anything in bad taste.

"Of course, if you were putting a line of clothing together for just another men's store, then you wouldn't have that approach at all. Because the first thing someone would say is, 'Wait a minute, maybe you don't like it, but there is a customer who likes green suits, and we have to have a couple of green suits.'

"Well, we don't have green suits. And we don't spend any time discussing it.

"And there are people who still wear double-knit suits - polyester double-knit suits - but we don't carry double-knit. Oh, I'm sure that in Texas we could sell some doubleknit suits. But we don't spend a lot of time talking about the sociological reasons for not doing it. It's not our custom.

"Some of this probably sounds smug. But it isn't smug - because we take it very seriously."