Task: Question Generation
Input (English): Passage: Handel joined the Hamburg opera house when it was experiencing a period of considerable artistic success. This blossoming followed the arrival of Reinhard Keiser, who had become musical director at the Gänsemarkt in about 1697, and in 1703 succeeded Johann Kusser as the theatre's manager. Born in 1674, Keiser had studied under Johann Schelle and probably Johann Kuhnau at the Thomasschule zu Leipzig.  In 1694 he was employed as a court composer at Brunswick, where in three years he composed seven operas, at least one of which (Mahumeth) was performed in Hamburg. According to Handel's biographer Donald Burrows, Keiser was a good judge of popular taste, with a flair for writing Italian-style arias.  Between 1697 and 1703, prior to Handel's arrival, about a dozen more Keiser operas had been staged at the Gänsemarkt. Despite his on-stage successes, Keiser was an unreliable general manager, with expensive private tastes and little financial acumen, often at odds with his creditors.It is possible that Keiser, who had connections in the Halle area, had heard of Handel and was directly instrumental in securing the latter's post in the Gänsemarkt orchestra; certainly he was a considerable influence on the younger man in the three years that Handel spent in Hamburg. Another important Gänsemarkt colleague was the house composer and singer Johann Mattheson, who noted Handel's rapid progress in the orchestra from back-desk violinist to harpsichord soloist, a role in which, said Mattheson, "he showed himself a man—a thing which no one had before suspected, save I alone". Mattheson was less complimentary on Handel's early efforts at composition: "He composed very long, long arias, and really interminable cantatas", before, it seems, "the lofty schooling of opera ... trimmed him into other fashions".
Output (English): What is the name of the person who "showed himself a man—a thing which no one had before suspected" while serving in the role of harpsichord soloist, according to Mattheson?