How did thematic transformation change musical structure in the works of Berlioz and Liszt?

In the works of Hector Berlioz and Franz Liszt, thematic transformation fundamentally altered musical structure by shifting the basis of organization away from the Classical sonata form principle—which relied on the contrast, conflict, and reconciliation of keys—toward structures unified by extramusical programs or poetic ideas.1

For Berlioz, the idée fixe served as a structural device that recurred throughout a work, taking on different characters in each movement to reflect the narrative "plot" of the symphony. While this provided musical and dramatic cohesion, it functioned outside the traditional abstract confines of the sonata.213

Liszt expanded this concept into a core principle of musical organization, particularly in his symphonic poems and his Piano Sonata in B Minor. By using a single theme in vastly different guises, Liszt could base an entire work on the transformation of that material rather than on key-based development. In his sonata, he utilized a single extended movement with subdivisions analogous to sonata form, but the free transformation and combination of his four themes departed from the standard Classical order. Ultimately, this approach prioritized thematic unity and programmatic narrative over the traditional tonal architecture of the Classical era.1425