 However, in September 1739 he was expelled from the university for poor attendance, having "hardly attended Natural Science more than once or twice".In 1740, Mozart began his career as a professional musician, becoming violinist and valet to one of the university's canons, Johann Baptist, Count of Thurn-Valsassina and Taxis.This was also the year of his first musical publication, the six Trio Sonatas, Opus 1.