Of all the great and much-loved composers, Schubert is perhaps the least known. This season, as the Chicago Symphony Orchestra explores the genius of Schubert’s output, most still come to this music with a shadowy portrait of the man and a surprisingly incomplete picture of his life’s work beyond the familiar landmarks — the Unfinished and Great symphonies, the Trout Quintet, a dozen or so songs, and a handful of other pieces.

Theodore Thomas, founder and first music director of the Chicago Symphony Orchestra, programmed both the Eighth and Ninth symphonies (the Unfinished and the Great) in its very first season, 1891-92. Those two works were regularly played during the CSO’s early years, and when Orchestra Hall was built in 1904, Schubert was one of just five composers whose names were engraved on the Michigan Avenue façade — his place essentially guaranteed by these two symphonies alone.

Schubert’s other symphonies took much longer to be added to the CSO’s repertoire: the Third only in 1960, the Fourth the following year, and Symphony No. 1 not until 1982 — the CSO’s 91st season! (By then, even all Mahler’s symphonies, including the incomplete Tenth, had been played here.) Presenting all Schubert’s symphonies in a single season, as Riccardo Muti does this year, is a CSO first. (The orchestra worked its way through all nine Beethoven symphonies, to make the most obvious comparison, in its first three years.) Clearly, Schubert’s time is overdue.

Schubert died the youngest of all the major composers — at 31, he was even younger than Mozart — and that fact, coupled with the limited popularity and success he enjoyed during his brief lifetime, made him music’s textbook case of neglected genius, tragic early death and unfulfilled promise. Even Franz Grillparzer’s often quoted epitaph — “The art of music here entombed a rich possession, but even fairer hopes” — engraved on the cemetery monument erected in 1830, just two years after Schubert’s death, encouraged that opinion.

Our familiar picture of Schubert is not inaccurate as far as it goes. Quiet, shy, and diminutive (the military rejected him because he was barely 5 feet tall), with poor eyesight (he wore his gold-rimmed glasses to bed), Schubert was never a commanding or charismatic figure. He had none of the ambition or savvy of Beethoven, his senior by 26 years, who took Vienna by storm before Schubert was even born. But in the years after his death, Schubert was turned into a cardboard caricature — a poor coffee-house composer who jotted down songs, one after another, on the tablecloth or the back of a menu, while the world passed him by. In Das Dreimädlerlhaus  (“The House of the Three Maidens”), an operetta about Schubert performed more than 80,000 times and translated into some 20 languages since its premiere in 1916, the composer loses his girl to his best friend. (Revised by Sigmund Romberg as Blossom Time, it was enormously popular in this country during the 1920s.)

Otto Erich Deutsch’s standard catalog, which ascribes a D number to each of Schubert’s compositions, itemizes nearly a  thousand works, beginning with D. 1, a piano-duet fantasy written in 1810, when Schubert was just 14. The statistics are impressive: seven complete symphonies (and fragments of another six), 22 piano sonatas, 15 string quartets, six masses, nine operas (and nearly as many left unfinished), and history’s most astonishing outpouring of songs — more than 600 in all. The Deutsch catalog represents just 17 years of constant, steady work. “When I have finished one piece, I begin another,” Schubert said of the pace he maintained throughout his career. Although inevitably uneven, the quality remains astonishingly high, and in the last year of his life, when he was possessed by a fierce creative energy, he produced a string of masterpieces — the C major string quintet, the three last piano sonatas, the E-flat mass, and Schwanengesang (literally, a swan song) — that is one of music’s greatest miracles. Yet very little of Schubert’s work is in the active repertoire today — a tenth perhaps, less than that of any other major composer — and we know virtually nothing of his sacred music or his operas, two genres to which he contributed profusely. The Mass in A-flat major, to be performed in February, is only the second time in 123 seasons that the CSO has programmed one of Schubert’s masses. (The E-flat mass was presented here in 1975.)

During his lifetime, Schubert was recognized as a natural talent — one of his first teachers said, “He has learnt everything from God, that boy” — and he was known as a composer to whom music came effortlessly and quickly. (It is relatively easy to calculate the speed with which he worked, because his composition teacher, Antonio Salieri, taught him to date his manuscripts.) In 1815, the peak of his productivity, he composed 145 songs alone, including “Der Erlkönig,” one of the greatest masterpieces in the literature. Perhaps one song (a setting of “Hark, hark, the lark” from Shakespeare’s Cymbeline) was, in fact, written spontaneously on the back of a beer-garden menu, as legend insists. But when Joseph von Spaun reported, clearly without evidence, that Schubert composed “Der Erlkönig” “in no time at all  . . .  just as quickly as one can write,” he only fanned Schubert’s reputation as a facile and largely intuitive composer, as opposed to a learned, hard-working, cerebral titan like Beethoven, who sketched copiously and sweated over every note. As late as 1894, in an influential essay, the British scholar and composer Hubert Parry could still maintain that Schubert had no feeling for “abstract design, and balance and order,” and “no taste for the patient balancing, considering, and rewriting again and again, which was characteristic of Beethoven” — a verdict that remained unchallenged well into the next century. (George Bernard Shaw called Schubert charming but brainless.)

Although Schubert was the only native among the so-called Viennese masters, he spent his whole life in Beethoven’s shadow. Much has been made of the fact that the two men were not friends, but in truth Schubert and Beethoven lived in the same city for years and never knew each other. Schubert made no secret of his fondness for Beethoven’s music; in 1822, he published a set of variations dedicated to Beethoven “by his admirer and worshipper, Franz Schubert.” We do not know what, if anything, Beethoven thought of Schubert’s music, except for the story, perhaps apocryphal, that when he was given several of Schubert’s songs only days before he died, he said, “Truly in Schubert there is a divine spark.”

On March 19, 1827, Schubert visited Beethoven for the only time. Beethoven was very ill and died just a week later; Schubert helped to carry the coffin at the public funeral. A year and a half later, Schubert’s own grave was dug in the same cemetery, separated from Beethoven’s by just three others. Five days before he died, Schubert asked to hear Beethoven’s C-sharp minor string quartet, and Karl Holz, one of the violinists who played it for him that week, later described this musical offering in words the rest of the 19th century would not forget: “The King of Harmony had sent the King of Song a friendly bidding to the crossing.”

The stature of these two figures was as different as their music. Beethoven was a celebrated, if controversial, performer almost from the day he made his debut in Vienna in 1795, and his compositions were internationally performed and widely published. But, in a city where music was played nearly every night, Schubert gave just one public concert devoted to his compositions in his entire career. He never achieved success in the opera house, which disappointed him greatly, and he never heard a professional performance of any of his symphonies. Only one of his compositions — the E-flat piano trio — was published abroad during his lifetime.

Yet, although his reputation was local, his renown in this important musical center was not negligible — a letter addressed to “Franz Schubert, famous composer in Vienna” reached him from Germany without apparent difficulty — and he was neither the outcast nor the miserably neglected talent that legend has made of him. Schubert may be the first composer in history to earn a living from his writing alone, without a full-time job (he stopped working for his schoolmaster father in 1818), patrons, or pupils to help pay the bills. He always had plenty of friends, and, right from the beginning, his surname was given to the celebrated evenings of music and socializing over which he presided from the piano bench. The Schubertiade became a favorite subject of early 19th-century painters, and these cozy parlor scenes, crowded with well turned-out men and women who appear to be utterly transfixed by Schubert’s music, ideally satisfied the Biedermeier passion for bourgeois conviviality. “When the music was done,” Franz von Hartmann remembered of an evening at Joseph von Spaun’s, “there was grand feeding and then dancing.”

Several years after Schubert’s death, Robert Schumann wrote, “He ought to have lived to see how celebrated he is today; it would have inspired him to his best.” Schumann had been one of the first to recognize Schubert’s genius. (At 18, he wrote him a fan letter, but unaccountably did not mail it.) After his celebrated discovery of the Great C major symphony in 1837, he said that anyone unacquainted with this work “knows very little about Schubert.” But Schumann himself knew only a fraction of Schubert’s music, and many more discoveries were to follow. Schumann did not live to witness the most impressive find of all — the two movements of the Unfinished Symphony that were performed for the first time in 1865.

Generations of musicians persisted in thinking Schubert the inferior of Beethoven, and, early in the last century, Arnold Schoenberg could still raise eyebrows by insisting on Schubert’s “inconceivably great originality in every single detail next to a crushing figure like Beethoven.” Schubert’s symphonies and piano sonatas suffered most, of course, by comparison; the brilliance of the songs only underlined his reputation as a gifted miniaturist. The sonatas in particular waited an inordinately long time to be taken seriously — in the 1920s, Rachmaninov was stunned to learn that Schubert had written piano sonatas at all — and the last three, now considered among the greatest ever composed, have only recently taken their place in the piano repertoire. (Both Mitsuko Uchida and Paul Lewis have devoted recitals to them here in recent seasons.)

Scholarship has, perhaps inevitably, turned its attention to Schubert’s sexuality, upsetting the conventional portrait of the composer as a genial bachelor, hapless in love, with evidence of his homosexuality. We have always known that Schubert was timid with women (he did not like to dance, partly because of his height) and he seemed to prefer the company of men. His fondness for the magnetic Franz von Schober, who favored exotic outfits and occasional cross-dressing, was apparently stronger and more genuine than his feelings for any of his female acquaintances. Despite his large and supportive circle, Schubert remained unattached while nearly all of his friends, like the upwardly mobile men who attended his evenings of music-making, took good jobs, married and started families. Schubert was very much a misfit in 19th-century Vienna.

Today we know more of Schubert’s dark side than any previous generation of music lovers. It gives us a context for the acute despair of Winterreise or the heartbreaking sadness in the Adagio from the C major string quintet — music that once seemed completely at odds with the composer of the genial Trout Quintet. When Maurice Brown published his classic “critical” biography in 1958, he remarked that “Schubert has been for 30 years, and will be for 40 years to come, spared the attention of centenary celebration, with its unavoidable exaggerations and distortions.” Brown never guessed how dramatically the familiar features of this famous composer would continue to change. (A life mask, apparently suppressed by the composer’s friends for many years, reveals a strong and determined profile, not unlike Beethoven’s.)

Today, we are closer to seeing Schubert whole than ever before. The music continues to tantalize us with a richness and ambiguity of expression and an emotional sensitivity that cannot be explained by historical fact or biographical detail. Perhaps the only conclusion we can draw, as we listen in wonder at his endlessly revealing output, is that Schubert understood more about the human condition than we will ever know about him.

Phillip Huscher is the program annotator for the Chicago Symphony Orchestra.