These include a fluid form of phrasing, in which each motif emerges from the previous one without interruption, the practice of letting accompanying material evolve into melodic material, and a kind of "Classical counterpoint" in which each instrumental part maintains its own integrity. These traits continue in the many quartets that Haydn wrote after Op. 33. In the 1790s, stimulated by his England journeys, Haydn developed what Rosen calls his "popular style", a method of composition that, with unprecedented success, created music having great popular appeal but retaining a learned and rigorous musical structure. An important element of the popular style was the frequent use of folk or folk-like material (see Haydn and folk music). Haydn took care to deploy this material in appropriate locations, such as the endings of sonata expositions or the opening themes of finales. In such locations, the folk material serves as an element of stability, helping to anchor the larger structure. Haydn's popular style can be heard in virtually all of his later work, including the twelve "London" symphonies, the late quartets and piano trios, and the two late oratorios. The return to Vienna in 1795 marked the last turning point in Haydn's career. Although his musical style evolved little, his intentions as a composer changed. While he had been a servant, and later a busy entrepreneur, Haydn wrote his works quickly and in profusion, with frequent deadlines. As a rich man, Haydn now felt that he had the privilege of taking his time and writing for posterity. This is reflected in the subject matter of "The Creation" (1798) and "The Seasons" (1801), which address such weighty topics as the meaning of life and the purpose of humankind and represent an attempt to render the sublime in music. Haydn's new intentions also meant that he was willing to spend much time on a single work: both oratorios took him over a year to complete. Haydn once remarked that he had worked on "The Creation" so long because he wanted it to last. The change in Haydn's approach was important in the history of classical music, as other composers were soon following his lead. Notably, Beethoven adopted the practice of taking his time and aiming high. Anthony van Hoboken prepared a comprehensive catalogue of Haydn's works. The Hoboken catalogue assigns a catalog number to each work, called its Hoboken number (abbreviated H. or Hob.).