'Tis a gift to be simple, 'tis a gift to be free,

'Tis a gift to come down where you ought to be . . .

Aaron Copland did not write the melody associated with those words, but if any tune belongs especially to Copland, this is it. He has used it so eloquently -- in the climactic variations section of "Appalachian Spring" and in a plain, powerful arrangement for solo voice -- that it is forever associated with his name.

But the music is not the only part of the old Shaker hymn "Simple Gifts" that is thematic in the life and work of Copland. The words are no less important. This composer, who reaches his 85th birthday Thursday, attained his unique position as the No. 1, certified national monument in American classical music by mastering the gift to be simple.

Or at least to appear simple, to generate an uncomplicated, spontaneous response. Copland is neither the most inventively and distinctively American of American composers (that would probably be Charles Ives or perhaps George Gershwin), nor is he the most influential on the world's music scene (that might be Elliott Carter, John Cage or Phillip Glass, depending on the kind of influence you are looking at).

He was by no means the first composer to work American folk motifs into classical structures -- that technique is at least as old as the American republic, and those who did it well before Copland include such relatively unknown composers as James Hewitt and Elie Siegmeister as well as the familiar giants Louis M. Gottschalk, Ives and Virgil Thomson.

But Copland is the American serious composer who has most thoroughly and durably captured widespread popular attention. After experimenting with other idioms that ranged from jazz to atonality, he compounded what we might call the "American sound" from a combination of folk melodies and French-inspired orchestration. He has set to music American folk heroes who range from Abraham Lincoln to Billy the Kid, not to mention more generic cowboys and Appalachian farmers. And he has done it for audiences that number in the millions. Most of his best known music is for ballet, which plays well on television and has helped him to win a wide video audience. But ballet in America was an esoteric art for specialized audiences when Copland wrote that music, and he has helped to bring it into the American cultural mainstream.

It is easy, looking at his most popular work, to dismiss Aaron Copland as a "cowboy from Brooklyn," the Walt Disney of American music, but there is much more than that in him and in his music. He has composed works of deep complexity and high quality in the most uncompromisingly modern idioms. It is excellent music though not frequently performed.

Much of it was written for piano, including the Variations (1930), the Sonata (1941), the Fantasy (1957). There is also distinguished chamber music: the Sextet (1937), the Piano Quartet (1950) and the Nonet (1961), as well as orchestral music with sounds that match such arcane titles as "Connotations" (1962) and "Inscape" (1967). In vocal music, there are the "12 Poems of Emily Dickinson." Some of this music may be destined to keep Copland's name and work alive a century from now.

It would not be quite accurate to call these works the "unknown" Copland; almost everything he wrote has been recorded, and his less-known works are generally well regarded by connoisseurs. But compared with "Appalachian Spring," "Billy the Kid," "Rodeo," "A Lincoln Portrait" and "Fanfare for the Common Man," this music has not found much of an audience. Once or twice, when invited to guest conduct an American orchestra, Copland has been known to think about programming "Inscapes" or "Connotations," then to dismiss the idea with the sad comment that it would be "too complicated."

"Appalachian Spring" is not too complicated, but there is a fine sophistication in its structures and its subtly shifting instrumental colors that works on the audience's imagination without calling attention to itself. It sounds utterly plain, like the lives of the Pennsylvania farmers who are the music's subject. But it is (like so much of Copland's best known work) highly skilled theatrical music, as subtly inflected as a fine actor's voice delivering a line of Shakespeare.

Copland's craftsmanship is of the highest order: the art that conceals its artfulness. And he has described it with a properly sophisticated air of simplicity. The art of orchestration, he once told an interviewer, "consists in keeping instruments out of each other's way." But, being Copland, he did not say "the art of orchestration," he said "orchestral know-how." Copland is a man who uses simple words where others use jargon, a man who understates his cleverness where others try to proclaim theirs.

Nowhere is the art that conceals itself more vital than in movie sound tracks. Copland composed eight of these, mostly in the 1940s, and he won an Oscar in 1950 -- ostensibly for "The Heiress" but actually for revitalizing the whole art of sound-track composing, bringing to it a new sense of freedom and stylistic variety. The award was well earned.

Copland talked about sound tracks in one of the best sequences of "Aaron Copland: A Self-Portrait," an unusually fine documentary that was aired last month on PBS. And he got to the essence of it: the way the music works subliminally on an audience, telling it how to feel about what is happening on the screen. There was one highly dramatic scene in "The Heiress" where the audience laughed at a sneak preview. Copland told how the studio executives came to him in a panic asking him to fix it. And the insertion of music to tell the audience that this scene was tragic, not funny, did the trick.

The documentary also explored other relatively unfamiliar sides of Copland's art and personality, revealing the expert hand of Joan Peyser (a musicologist at Yale and collaborator on Copland's autobiography in progress). Copland worked hard as a promoter of American music through concerts, festivals, publication of scores, articles in Modern Music magazine and such books as "Our New Music," "Music and Imagination" and "What to Listen for in Music."

He was associated with the summer school at Tanglewood for a quarter-century (most of that time as its chairman), and he has served as a mentor for younger composers as diverse as Leonard Bernstein, Ned Rorem, David Diamond, David del Tredici and Toru Takemitsu, encouraging each to develop his talent in his own distinctive way, not shaping them into little Coplands. In this, he is faithful to the example of his teacher, Nadia Boulanger, who became the godmother to a whole generation of wildly diverse American musicians largely because Aaron Copland was one of her first students.

Beneath the simple exterior of this visionary artist and solid craftsman, who was born when the century was new and who stamped his image firmly on its music, lie complexities that his mass audience has hardly begun to explore. If there is one tragedy in his life, it is probably that he has not managed to bring that audience along into the more rarefied regions of his art. But the fact that he found that audience and took it as far as he did makes him unique among the composers of his time.