As a composer and a critic throughout a life that nearly spanned the 20th century, Virgil Thomson helped American music find its own voice and its own audience. His music has often, and correctly, been called "witty," though he once said, "I don't think music's ever very funny." It has also correctly been called "cosmopolitan," but it is rooted in and draws its strength from the music he heard in childhood: hymns that he used to play as an organist in the Southern Baptist church his family attended; popular songs of the 19th century; the jazz and ragtime that were intensively cultivated in Kansas City, Mo., where he was born in 1896. Thomson, who will probably be remembered as much for his prose as for his music, brought his hometown with him to the larger, more sophisticated cities he inhabited for the rest of his life: Boston in the early 1920s, Paris (the Left Bank) from 1925 to 1940 and New York (the Chelsea Hotel, where he became a national landmark) from the 1940s until his death Saturday. He was a man of great vitality and broad interests, numbering among his acquaintances Jean Cocteau, James Joyce, F. Scott Fitzgerald, Andre Gide, Pablo Picasso, Christian Dior, Gertrude Stein (whom he set to music) and Ernest Hemingway (whom he did not set to music but whom he resembled, stylistically, in some ways). As a student of Nadia Boulanger in Paris (one of the first, with Aaron Copland), Thomson mastered the complex, eclectic forms of 1920s avant-garde music, but he mixed grass-roots American material into whatever he touched. A good example of his all-encompassing mixtures of sources and styles is the Sonata da Chiesa (1926) for clarinet, trumpet, viola, horn and trombone, produced under Boulanger's tutelage. The first of its three movements portrays a service he had once heard in a black church in Kansas City -- the preacher's sermon as well as the congregational chorale. The second movement is a tango (later used in his ballet "Filling Station"). He finally gets to a traditional European form, a fugue, in the third movement, but the fugue subject is a modified echo of the earlier tango. Similar grass-roots inspiration can be heard in his Symphony on a Hymn Tune and in numerous organ works, film scores and, perhaps above all, his operas. One of them, "Four Saints in Three Acts," is probably the Thomson work for which critical recognition and public attention have harmonized most closely (particularly since the excellent Nonesuch recording of a few years ago), but all of his music is readily accessible -- when it can be heard. Performances are relatively rare, considering the composer's reputation, and except for the most popular soundtrack suites, "The River" and "The Plow That Broke the Plains," none of his music is available in more than one recorded performance. Much of it is not available on records at all. At the New York Herald Tribune from 1940 to 1954, Thomson introduced to critical prose the directness and apparent simplicity he had cultivated in his music. He held that only practicing musicians should write music criticism (though he acknowledged the risk of conflicting interests), but he focused on readability and avoided technicalities and "insider" writing. His criticism -- available in four books and part of a fifth devoted to his Herald Tribune work -- is devoid of pretentiousness and gratuitous displays of erudition, which were very common at the time. His style may sometimes have been a shade too slangy, a point that becomes more evident as the years go by and terms such as "swell" or condescending references such as "the Philharmonic boys" begin to seem quaintly archaic. But it is enormously preferable to the pomposity that has often masqueraded as profundity in this tricky, demanding branch of journalism. He also cultivated firm, vigorous opinions expressed without ambiguity -- opinions that were frequently directed against the musical establishment of the times: institutions, performers and repertoire. Thomson was not one to unquestioningly accept prevailing opinion on the enthroned "masterpieces" of the standard repertoire (which was much smaller in the 1940s and early '50s than it is today), and he tended to focus his attention more on the music than the performance. But though he was an untiring advocate of contemporary music, particularly American music, he did not accept it uncritically either. He used to refer to some of his more experimental contemporaries as "the far outs," and was often highly critical of 12-tone composers such as Arnold Schoenberg and Milton Babbitt, describing the Schoenbergian 12-tone row as "merely a rule of thumb to make atonal writing easy." But he also expressed strong reservations about a work as fresh and accessible as "Porgy and Bess": "Its faults are numberless; but its inspiration is authentic, its expressive quotient high," he wrote. Of the "far outs," he had a strong mutual admiration for John Cage, who was unlike Thomson in his taste for radical experiment but like him, both musically and personally, in an essentially childlike simplicity underlying a chic exterior. Besides composers ranging from Rossini to Sibelius, he was unsparing in his criticisms of such idols and sacred cows as Arturo Toscanini, Jascha Heifetz, Artur Schnabel, the New York Philharmonic and the Metropolitan Opera, sometimes for what he considered unmusical distortions in performance and sometimes for devoting so much of their time to what he called "dead music" -- an expression essentially synonymous with enshrined (or embalmed) classics. "Dead music," he once wrote with the special irony of a living composer, "is very beautiful sometimes and always pretty noble, even when it has been painted up and presented by the undertakers who play or conduct it with such solemnity at our concerts. Live music is never quite that beautiful." But apparently he had ambivalent feelings about prestige and popularity. Ned Rorem, a fellow composer, student and protege, quotes him as saying once, at a party where the hostess was upset that he was not getting much attention: "When I find myself among those who don't know my name, I know I'm in the real world." Rorem's admiration for him is comparable to that of Leonard Bernstein's for Aaron Copland, though it is more impressive to hear the superlatives from Rorem, who does not usually become so enthusiastic in print. He has called Thomson the "best of all possible songsmiths" and stated flatly that "Thomson as critic was the best in the world" -- a statement many other critics would accept. Rorem, coming to New York to study music in 1944, found himself embarked on what he later called "the adventure of Virgil Thomson." He became Thomson's copyist "in exchange for $20 a week and orchestration lessons," and years later he recalled that "during the months I worked with Virgil I learned more than during years in the world's major conservatories. ... Virgil is nothing if not lucid: I can recall today as on a record each word he spoke 35 years ago. "His lucidity is due no less to an innate clarity of mind than to a voicing of that mind through an ideal language of economy: He speaks French in English. Since he knows what he's talking about and doesn't waste words, merely to be in his presence is to learn. "His music resembles, more demonstrably than with any composer I know, himself. It is impatiently terse, strong but free of fat or padding, sensuous without self-indulgence, and we absorb it like a cold acid which bathes a core of hot beauty. His music is also very, very witty -- if that adjective makes sense when applied to non-vocal works." If instrumental music can be witty, Thomson has certainly written witty instrumental music. A section of his soundtrack suite from "The River," for example, soberly titled "Industrial Expansion in the Mississippi Valley," opens with a jazzy transcription of "There'll Be a Hot Time in the Old Town Tonight" and moves on into what sounds like an orchestrated riot. Elsewhere, he makes witty use of music that calls to mind words as varied as "Jesus Loves Me" and "Ach, du lieber Augustin." He also showed a special kind of homespun dry humor in much of his writing and occasional offhand remarks. "I have never been psychoanalyzed, so I have no guilt," for one. "The press is beginning to discover that I am a conservative composer because my music is quite often grammatical," for another. And, of course, "The ideal listener is one who applauds vigorously."