Eve Stillman, lingerie designer and president of the firm that has her name, was helping a customer on Fifth Avenue make a choice from her collection of lace-trimmed, satiny lingerie. When the customer handed a saleswoman the several items she wanted to purchase, Stillman suggested that some of the garments might be too small. "Makes no difference," smiled the woman. "I just leave it at the end of my bed."

Whatever the reason - the sensual appeal to the wearer or the seductive attraction of the beholder - silken, slippery satin, lace-trimmed lingerie and sleepwear have been snapped up by women here and in stores across the country.

"It's the feel," says Monica Greenberg, wife of a Washington builder. "Outwear you wear for other people. These things you wear to make yourself feel good. You feel as well-dressed from the inside as you do out."

At Woodward & Lothrop, lingerie sales, even in junior departments, are up about 25 per cent; Hecht's says lingerie business in its Tyson's Corner branch for day wear alone (as opposed to things designed to sleep in) is 30 per cent ahead of last year. Elizabeth Arden has done twice the lingerie business of a year ago and before Christmas couldn't keep anything in stock that was trimmed with maribou.

And the appeal of things that simply feel good to wear doesn't stop with intimate apparel. Saks Fifth Avenue sold hundreds of cashmere robes at $160 and could have sold three times that, according to Sally Frame, a Saks merchandise manager.

Although customers were slower to accept the silk lingerie in Crevy Chase than they were in their branches in other cities.) Calvin Klein sold out of his white cashmere peasant style sweaters with a price tag of $100 in a month: Garfinckel's sold 2,400 cashmere turtle neck and cowl neck sweaters for Christmas.

Luxurious silk shirts have been snatched off the racks in Washington about as fast as they've been put up for sale. Stephen Karel, vice president of Irka, which import crepe de chine shirts from Hong Kong, has been selling one of his styles (the basic shirt) at the rate of 2,000 per month. Satin sheet sales are up too; double in fact at Bloomingdale's where Vice President lester Gribetz (who has never slept on them) says the customers are young and old, male and female.

All of which raises the baffling question that chronically besets an industry whose success depends on correctly reading the mood of the American public: why this trend at this particular time?

And the puzzle that jounalists, sociologists and historians love to ponder: what, if anything, does it mean?

"In the past, if you wore a black satin undergarment," says columnist and commentator Dr. Joyce Brothers, "it was because you were hoping to have an affair. Now you wear it because you like to feel sexy, not because you have any plan in mind."

Self-indulgence may, in fact, be a trade mark of the mid-'70s, according to Brothers and others who make a living observing the evolution of the nation's mind-set.

Author Tom Wolfe labels this the "Me Decade," and introspective era paid for and encouraged by 30 years of uninterrupted post-war affluence in the United States. The dream, he says, has become "changing one's personalty - remaking remodeling, elevating and polishing one's self . . . and observing, studying and doting on it."

He sees its roots in the '60s partly in the hippie, psychedelic phenomenon, the women's movement (which elevated "an ordinary status - woman, housewife - to the level of drama"), and the New Left students, whose "costumery tended to be semi-military: the noncom officers' shirts, combat boots, commando berets - worn in combination with blue jeans or a turtleneck jersey, however, to show that one was not a uniform freak."

All were part of a trend toward asserting one's self against established ways of thinking, acting - and dressing. And if clothes in the '70s do notmake the political statements they did 10 years ago, the lesson learned then - roughly translated as: If it feels good, wear it - lives on.

Thus, Bergdorf Goodman, which has always had a few customers for silk stockings, sold out of them last fall, and by this fall plans to add silk pantyhose from Paris.

"For some the appeal is the sexy look, complete with garter belts. But the feel of the silk is really incredible," says Harvey Belsano of Bergdorf's, who expects to sell cashmere and silk blend pantyhose, too, at $40 a pair.

But Amitae Etzioni, professor of sociology at Columbia University, takes a dimmer view of this hedonistic drive for clothes that are pleasing to the touch.

"When a society falls apart, people fall back with a growing celebration of the self, of the body," he says.

He finds it totally in keeping with the dissolution of the family in America or cheating in universities. "Such things are rampant in this country," he comments. "Taking shortcuts, bending rules. . ."

The fashion industry's approach to the "sensual '70s" is more prosaic, but no less compelling for those involved in it. When designers and merchandisers look at the market for luxury items, they see practical fashion factors at work.

The synthetic fabrics of a few years ago are part of the reason. When bras and slips went out, polyester double-knits turned out to be uncomfortable against the skin. And slippery nylon shirts, which didn't feel bad, proved to be clammy or cold at the wrong time.

Later, energy shortages pushed up the price of clothes made of petroleum-based synthetics. Suddenly luxury fabrics didn't seem so far out of reachZ.For a generation that had never worn silk shirts and had never seen satin underwear, sensually appealing clothes were affordable. Luxury and quality became sought-after characteristics rather than instant (and quickly passing) chic. And as prices continued to rise, owning fewer, better things replaced owning lots of things.

All of this blends with the technical opinions of leading designers whose professional lives are spent judging quality.

"I pick my fabrics first by how good they feel," says Seventh Avenue designer Geoffrey Beene. "Then I pick the colors and patterns."

Bonnie Cashin agrees. "At least half of the appeal of clothing is the sybaritic quality." Cashin, who says she's always felt that way about fabrics, has already sold all the cashmere she was able to order from Scotland for this fall.

"Real fashion is to feel like a woman," says Eve Stillman, who sold thousands of silky nightgowns at $50 to $100 this winter. (She thinks that most of her customers "wear lingerie only as a prelude; they sleep in the nude.")

Beatrice Coleman, president of Maidenform, thinks some women wear lacy underwear just to contrast it to their very tailored suits. But even those man-tailored suits andseparates are softening up and becoming far more soft and feminine, partly because less complicated construction i cheaper but also because of the shift in tastes. "Women no longer need to prove their place in business so they don't have to wear trousers or pull their hair back severely," says Saks' Sally Frame. "Guys are used to seeing women in business. So now women can be more feminine inwhat they wear."

Frame believes this is probably why women are now seeking out the bras that give some uplift to their figures. "Not the old bullet-proof variety (of bras)," laughs Frame, "but soft, underwire styles that give them some cleavage."

What's next?

Joyce Brothers thinks the appeal of such items as satin lingerie will pass only because in fashion, nothing continues forever. "The senses adapt and so you will need a rougher texture so that you can again be aware of the feel of something," she says. (KEY OFF)(KEYWORD)s a rough guess, she sees a texture like brushed cotton as a successor to the slippery satins, "to reawaken the senses when they get used to satin."

It's a very good guess. French designers have started to introduce "pilou," the cotton flannel in baby blankets, for shirts and dresses. Geoffrey Beene has used printed cotton flannel for his next collection, and Ephraim Block, whose firm Perfect-Fit makes satin sheets, sees brushed cotton flannel as the possible next step.

Meanwhile, sensuous clothes for men may be gaining a foothold, too. Steven Karel of Irka, who wears a version of the silk crepe de chine shirt, plans to introduce a line of them for men in the fall. "Somehow, when I put on a silk shirt I feelmore important, more luxurious," he says.

Fernando Sanchez will also expand to a line for men including drawstring pants and loose, shirt-like tops, and cashmere or silk robes. Cotton sateen will be one of the options.

"When I put on a silk shirt with my old pants, it not only feels good, it makes me do things better," says Irka designer Bern Conrad. "Making me feel better about things . . ." he muses. "Maybe that's the very best thingclothes can be expected to do."

And the next question: What does that say about America in the '70s?