I decided to paint the celebration of the wonder of the world. I didn't want to paint depressing pictures. There were so many depressing things; so many self-conscious, forced, foolish things. That's why my paintings are all on the cheerful side -- I felt it was needed. There are the two sides, but it's hard to get in everything.

-- Marjorie Phillips, in an autobiography to be published next spring.

At the great and honorable age of 90, which she attains today, Marjorie Phillips will celebrate her birthday at a private party at the Phillips Collection. On this day, her great interests in life come together. Her own paintings will hang with the works of Bonnard, Renoir, Monet and the great artists of the world whom she has collected and encouraged.

It is time to ask: which did she prefer to be -- connoisseur or painter?

"No question," she says. "It's a tremendous thing to be painting yourself. It takes everything you have."

But she has always given more.

She and her husband, Duncan, founded the Phillips, the first museum of modern art in the United States. She served as its associate director (1925-1966) and director (1966-1972). Their influence in the 20th-century art world was enormous. They bought paintings, deferring to each other's judgment. They encouraged young artists. Their eye for color and size and juxtaposition in exhibiting art made the Phillips Collection an almost secret, intimate treasure house of art for the knowing. And through it all, Marjorie Phillips kept a steady eye out for "artistic elegance . . . the essence of the thing."

Marjorie Phillips has been given advantages: Money, energy, talent and family. But though they helped alleviate the problems, nothing bought her immunity from tragedies: the death in 1966 of her husband, who was almost an alter ego; the almost lifelong illness of her daughter, Mary Marjorie, and of a brother, William Acker; and now, the infirmities and indignities of age.

"I think I have learned more in this past year, at least about the kindness of people, than in all my life before," she says.

She lives very grandly, with a full staff, in a landmark house designed in 1929-30 by John Russell Pope, on 18 acres on Foxhall Road, one of the highest taxed properties in the city.

To save strength for her birthday, she recently talked in her bed-sitting room, the one with the view of the treetops between the house and the Potomac. Above the fireplace was a painting she once did of the view. She sat in a fat easy chair, stuffed to cuddle her thinness. She had on a quilted robe with a soft, frilly collar. Here and there through the house, near where she sat, were small electric heaters.

Though family and friends abound, many old ones are gone.

When her longtime chauffeur died a year or so ago, she told a friend she was going "to hire one young enough to last for the long haul."

She still mourns various dogs, especially the three poodles who lived to be 17 years old. A painting of her husband, finished many years after his death, includes the poodles. She has a new dog now, a schnauzer who is a great joy.

Around her she's built a serviceable U of desk, low cabinets and television stand, where she works at finishing her second book, an autobiography in words and pictures. Sunday she watched the presidential debates.

"I thought they both did very well," she said. "But I'm going to vote for Mr. Reagan."

Her secretary, Barbara Gasper, produces a letter from the president, congratulating her on her birthday. "My family don't agree with me," Phillips says.

Her son Laughlin later says, "My father was very liberal. But I suspect she was always more conservative than we knew. She's been much more forthright about her own opinions after my father's death."

Her own paintings line the walls of her downstairs hall, on both sides of the ping-pong table (a lure for the grandchildren). Many of the pictures have been rescued from the dust of the attic and the mold of the basement. (In the search, a Matisse is said to have been rescued from a closet.) Others are in the collections of the Whitney Museum, the Corcoran Gallery and Yale University.

A few, including a self-portrait, hang in the drawing room. The great city and river scene by Oskar Kokoschka hangs over the fireplace. Marjorie Phillips as a 1921 beauty, a wedding present painted by her uncle, Gifford Beal, is over the piano. "The Tablecloth," one of four by Pierre Bonnard she owns, is on the south wall.

In her study, with its wonderful vintage midcentury modern furniture, hangs granddaughter Liza's painting of an Atlantic seascape.

"We have Liza's Atlantic on the east end of the house and John Marin's Pacific on the west," her grandmother says.

Like other young women of good family of the period, Marjorie Acker lived at home (in Ossining, N.Y.) until she was married at 27. Her uncles, Gifford and Reynolds Beal, professional artists, had a studio at her grandparents' house. They encouraged her to be an artist, though her father opposed her studying art because she was a girl. She and her sister studied at the Art Students League in New York. They alternated years so one would always be home to keep her mother company and help with the children.

Marjorie Acker met Duncan Phillips in January 1921.

"I knew," she says, "I knew that art had the same tremendous meaning for both of us." When she should have been getting ready for her wedding, she spent her time painting the Mare'chal Niel roses he sent her.

When she came to Washington, ladies of her status, whether painters or not, had a busy, formal social life, paying calls and leaving cards. Her mother-in-law would sometimes excuse her with the explanation "Marjorie is sketching," as though that were more respectable than painting.

Whatever the demands of family and museum, she has always reserved time to herself to paint. She still has a large studio, adjacent to the study where her husband once wrote his books of art criticism.

Like many other strong-minded (as they were called) women of her era, she was adept at figuring out ways to do what she wanted to, while accommodating others.

"I suppose my house has suffered for it," she says, with a rueful air, obviously mock. "As for the children, well, I always had good help."

In her autobiography she writes:

I sometimes used to feel a little guilty about painting. But I was an artist when I married and I trusted myself. Of course Duncan knew that I was a painter and knew that I would continue to paint. He encouraged me to continue . . .

Some of my paintings he loved just as they were, but he couldn't resist making a suggestion sometimes. I didn't always follow his suggestions. Only if I agreed. I had to be strong.

Laughlin Phillips remembers when he was a child that, no matter what, in the morning his mother went into her studio and closed the door. But also, when he was hardly old enough to hold a brush, she taught him to use it and to mix paints. She taught her husband to paint and mixed his paints for him. When they traveled, as they often did, she would paint. When she took the children on picnics, she would paint.

Her favorite painting, "Summer Morning," to be on the cover of her autobiography, shows a nurse holding Laughlin, while she watches Mary Marjorie ride a pony.

Her husband was very fond of baseball, which she cared nothing about. She went with him carrying a sketch pad. Two of her baseball paintings have been widely acclaimed. The stance of the players, the crowds and the lights are so accurate that the Baseball Hall of Fame tried to buy the one called "Night Baseball" showing Joe DiMaggio.

"I doubt she knew who won," her son says.

Away from painting, she had a full other life. She and her husband were interested in politics and world affairs and had quite a salon. Laughlin Phillips remembers Sunday luncheons "when I would rather be outside playing, but someone important would be coming." Someone could include Matisse, Gertrude Stein, Walter Lippmann -- or other artists, writers, journalists, diplomats.

Though he remembers being very protected as a child -- that was the era when people worried about the Lindbergh kidnaping -- Laughlin Phillips remembers his family as being liberal with him. He did many other things -- worked for the CIA, founded Washingtonian magazine and refused to join the staff of the Phillips, though he became, at his father's death, its president.

In 1972, at a board meeting, to Laughlin's surprise, his mother announced she was retiring, and asked him point blank if he would take over as director. To her surprise he said yes.

My days were spent juggling different roles: painting in the mornings; running the house; being a mother; fulfilling social responsibilities, which we kept at a minimum; and, after Duncan's death in 1966, running the gallery until Laughlin took over in 1972. I didn't mind it because I had so many satisfactions. I was happy as long as I had some time to paint every day.