He was only 31 when he died, younger even than Mozart. And yet his death mask, molded sometime after 3 p.m. on November 19, 1828, is that of a man in his late 50s: puffy, furrowed, worn. It's the imprint of someone who has spent years - years far too early in life - looking at death head-on.

Which makes Franz Schubert, born in a suburb of Vienna 200 years ago this past Friday, the first modern composer: He was the first one who in music looked death in the face, faced the abyss, uncomforted by visions of Elysian fields or angelic choirs.No, the way to death at the end of Schubert's song cycle "The Winter Journey" ("Die Winterreise") is pointed by a pathetic old organ grinder. His feet bare on the ice, dogs snarling around him, he staggers to and fro, his raspy little ditty repeated again and again in the piano accompaniment. No one wants to hear him, and his plate stays empty.

Not until the plays of Samuel Beckett is there another scene of such emotional desolation. What Schubert presents us is an existentialist's definition of death: the end of the possibility of becoming. He shows us terror in a handful of notes.

Schubert's closest friends were shocked and bewildered when they first heard the songs of "The Winter Journey," set to poems by Wilhelm Mueller and completed in 1827. How to reconcile the songs' darkness of vision and starkness of texture with the usually cheerful little man they loved?

Today, we tend to think of Schubert as the composer of some of history's friendliest, most tuneful music. There are those lovable art songs, telling of Gretchen at her spinning wheel, the perky trout in the stream, the shepherd on the rock. Then there are the bubbly tunes of the "Trout" Quintet (its fourth-movement variations based on the song), the cellos' graceful song in the first movement of the "Unfinished" B minor Symphony.

No composer ever had so boundless a gift of melody, whether for voices or instruments of the orchestra or even the piano. In less than two decades of composing, Schubert poured out more than 600 solo songs, and his lyric impulse gushed over into symphonies (eight finished, several more left incomplete), piano sonatas (a dozen, more or less, finished) and legions of smaller pieces, string quartets (13 completed) and on and on. How did one man, for half his career shadowed by ill health, do it all?

But there was a dark side to Schubert that surfaced in drunken binges and inexplicable, china-shattering rages. It surfaced, too, in his music, in patches of surprising violence, or at least disturbance. And then there was the unsettling testimony of the composer's friend Josef Kenner.

"Anyone who knew Schubert knows how he was made of two natures, foreign to each other," Kenner wrote, "how powerfully the craving for pleasure dragged his soul down to the slough of moral degradation."

Those are strong words, and they've kept Schubert scholars abuzz ever since. The most controversial interpretation was advanced as recently as 1989 by the musicologist Maynard Solomon: that Schubert, who never married or had a serious relationship with a woman, was homosexual.

What's pretty much beyond dispute is that the disease that haunted Schubert's last six years - with rashes and patchy hair loss and, at the end, headaches, vertigo and vomiting - was syphilis. Although his death was attributed to what today would be called typhoid fever, the diagnostic resources of the day were pretty primitive.

It can't have helped Schubert's general health that the standard treatment for syphilis in those days was mercury, a potent poison. (Antibiotics were still a century and more in the future.) Nor that he was almost certainly an alcoholic and a heavy smoker.

Does any of this matter? Does it heighten our understanding of Schubert's music to know - or at least speculate - that he may have been hiding something?

Well, yes, it does. For there are unsettling characteristics of his music that elude normal harmonic and formal expectations. But they seem clarified by a sense of dark forces bubbling beneath the pleasant surface of the composer's life.

The lyric impulse, the tunefulness, the gracious harmonies fit the pattern of Biedermeyer Vienna, with its emphasis on middle-class comfort and a touch of sentimentality.

But how are we to explain those moments in Schubert when something sinister seems to boil up - if momentarily - from unseen depths? How, for that matter, are we to explain Schubert's unpredictable modulations, often shifting to the very last tonalities we could have expected?

Consider the first movement of the great B flat major Piano Sonata. It opens with a placid-sounding tune to which we could imagine singing words like "My love's a red, red rose." But as the movement progresses, time after time, Schubert sets up certain harmonic expectations only to dash them with something wholly unexpected. And then there's that jumpy, defiant first ending, for which there's no preparation and no reprise. What does it mean?

In the music of Beethoven, 27 years Schubert's senior, we tend to find clearer-cut proclamations and oppositions.

Beethoven had a hard life, too, and we often sense his personal struggles in his music. Schubert's musical expression of personal conflict is subtler, more veiled.

Given the evidence of Schubert's personal life, Elizabeth Norman McKay (in a recent biography published by Oxford University Press) theorizes that the composer had a mild form of manic depression known as cyclothymia.

This would account for the dark moods, the irrational rages and sprees of spending and debauchery, but also for the mind-boggling fecundity of Schubert's "up" periods.

In so short a life, productivity won out over editing, and the Schubert catalog is littered with pieces begun but never finished. And by no means is everything in that enormous catalog a masterpiece.

But we do Schubert - and ourselves - an injustice if we judge his music by Beethovenian standards. Although the two composers shared something of the same artistic patrimony and lived at the same time in the same city, they were profoundly different men and creators.

With Beethoven there's always a sense of adventure. With Schubert's music, for all the subtle shifts along the way, it's as if we've always known it, as if we're tapping into a collective unconscious.

We love Schubert for his matchless gift of melody and his expressive subtlety. We clutch him to our hearts for those moments when he articulates our deepest disappointments in love and our darkest fears of death.

Schubert went to his grave without hearing a professional performance of one of his symphonies, and few of us today know more than a handful of his hundreds and hundreds of works. His operas remain unperformed and unknown. Only now is a project under way, on the Hyperion label, to record all 600-plus songs.

But is there anything in Western music more precious, more directly addressing the human condition, than "The Winter Journey," or the earlier song cycle "The Pretty Mill-Maid" ("Die schoene Muellerin")? Anyone who sits through a performance of either song cycle will leave a changed person; at the end, audiences tend to sit in interminable, stunned silence before daring to applaud.

That is genius. That, unsettling as it may be, is truth and beauty.