Everyone who loves modern art's old masters should see the John Quinn show, which will be on view all summer at the Hirschhorn Museum and Sculpture Garden. It opens with a masterful Cezanne and ends with a Matisse that is just as fine.In between are Duchamps, Picassos, Seurats and Brancusis. Quinn owned them all.

The more you see his art the less you understand him. Where did Quinn get his courage, his unerring insight, his acute prophetic eye?

His face provides few answers. John Quinn in his portraits looks like a Wall Street laywer, which is what he was.

He was not born to privilege; his father, an Irish immigrant, owned a little bakery in Fostoria, Ohio. Nor was he schooled by esthetes; he studied law at night here at Georgetown University. Some see in art collecting a path to immortality, but Quinn, who started buying at the age of 41, did not care for fame. He wasn't shy, but his motive wasn't ego. Unlike Barnes or Phillips, or Joe Hirschhorn for that matter, he formed no Quinn Museum. He didn't even catalogue his paintings, though he owned more than 3,000. He died in 1924. He will directed his collection be sold off, dispersed.

In it there were pictures that everybody knows now - Picasso's Blue Period "Guitarist"; Seurat's "The Circus" (Quinn left it to the Louvre); Duchamp's "Nude Descending a Staircase, Number One" and Rousseau's "Sleeping Gypsy," now at the Museum of Modern Art. Museums hate to lend works of art so famous, and these are not on view, but so rich was Quinn's collection, and so fine is the selection here, that they are hardly missed.

"The Noble Buyer: John Quinn, Patron of the Avant-Garde" was organized by Judith Zilczer of the Hirshhorn's staff. Visually and intellectually it is one of the best exhibits that museum has yet done.

We grew up on these oils, drawings and sculptures, and praising them is easy now. But Quinn acquired them when buying them took guts. More than a collector, Quinn truly was a patron. He preferred to buy living artists' art.

He helped support Brancusi, Picasso and Matisse. He bought the Cubists, the Vorticists of England, and the Italian Futrists. He bought from Irish painters, too, and from dozens of Americans. "He was," a friend recalls, "mad to mix with genius." That was as true in literature as it was in painting.

Quinn knew Ezra Pound. Through poet William Butler Yeats Quinn met his lover, Jeanne Robert Foster. Quinn defended in court those who dared to publish "Ulysses" by James Joyce. When he was not buying pictures, Quinn purchased autograph manuscripts by Joseph Conrad and T.S. Eliot.

"To miss modern art is to miss all the thrill and excitement that our present life holds," wrote Quinn in 1912. "I do not want to buy the ideals of artists of 50 or 75 years ago . . . I prefer the more exciting and interesting and profitable game of buying living art."

In 1913, Quinn helped organize the famous New York Armory show. Each evening after work he would visit it again. Before it closed he had spent more than $5,000 buying pictures from the walls.

At first his taste ranged widely. There was scarely an American avant-gardist whose work were not included in the Quinn collections. Later he was more selective, focusing on Picasso (he owned more than 50), Matisse, his beloved Brancusi, sculptor Duchamp-Villon, Rousseau and Seurat. Quinn was not buying "names." He refused to purchase Klee, Chagall, Modigliani; he relied upon his eye.

Quinn did not live in splendor. His pictures leaned against the wall of his New York apartment, where he showed them only to his closest friends. But he lent them generously to galleries and museums. As much as any man, John Quinn in the years after World War I helped make New York City a center of modern art.

There are, in the Hirshhorn show, both splendors and surprises. Most of us will greet the Picassos, Brancusis, Matisses and Duchamps as familiar friends. But Gwen John's works are less well-known. A gifted English intimist who painted subtle portraits in Vuillard-Morandi colors, John was the sister of flamboyant Augustus John. Diego Rivera is best known for his passionately political pictures, but Quinn bought his Cubist works. American Charles Sheeler, is known as a "precisionist," but Quinn bought his abstractions, one of which. on view here, seems a work of homage to Marcel Duchamp.

Not all these things are masterworks. There is a hugh Augustus John here that will set your teeth on edge. (The child in the foreground looks as if he's wearing someone else's head.) Quinn's Arthur B. Davies, also hugh, seems a pseudo-Cubist potboiler. But then Quinn was loyal to artists, not only art. Both Augustus John and Davies were among his friends.

Between 1924 and 1927, Quinn's art collection was dispersed at auction. The total take was something near $600,000, though some works went for next to nothing. A Walt Kuhn which Quinn had bought for $600 and sold for only $10.

No one now is sure exactly what Quinn owned, though the Hirshhorn's Judith Zilczer has spent more than three years trying to find out. So far she has traced 500 pictures that were in his collection. Of course, 79 have been assembled for this show.

Quinn, in his collecting, moved towards but never reached what we'd call pure abstraction. He said he enjoyed the Armory Show more than he would a visit "to an old cathedral. This thing is living: The cathedrals, after all, seem mostly dead . . . When one leaves this exhibition he goes outside and sees the light streaking up and down the tall buildings and watches their shadows and feels that the pictures he has been inside after all have some relation to the life and color and rhythm and movement that he sees outside." Like other great collectors - one thinks of the Steins, Walter Arensberg, Duncan Phillips, and more recently Paul Mellon - Quinn bought what he loved.

Even in death he supported the new art. Underground artist Marcel Duchamp lived for many years by slowly selling the Brancusis collected by John Quinn.

"The Noble Buyer: John Quinn, Patron of the Avant-Garde," accompanied by a good catalogue, will close Sept. 4.