A basic theme is reprised throughout a musical work, but it undergoes constant transformations and disguises and is made to appear in several contrasting roles.
However, the transformations of this theme will always serve the purpose of "unity within variety" that was the architectural role of sonata form in the classical symphony.
The difference here is that thematic transformation can accommodate the dramatically charged phrases, highly coloured melodies and atmospheric harmonies favored by the Romantic composers, whereas sonata form was geared more toward the more objective characteristics of absolute music.
[3] He may have already had experience in metamorphosing themes into various shapes in his early operatic fantasies and improvisations[4] and been also led to this practice by the monothematicism Liszt employed in many of his original works, including most of the Transcendental Etudes.
[5] Conservative critics in Liszt's time viewed thematic transformation as merely a substitution of repetition for the musical development demanded by sonata form.