Progressive rock (or art rock) grew out of the classically-minded strains of British psychedelia. In 1966, the level of social and artistic correspondence among British and American rock musicians dramatically accelerated for bands like the Beatles, the Beach Boys and the Byrds who fused elements of cultivated music with the vernacular traditions of rock. According to Everett, the Beatles' "experimental timbres, rhythms, tonal structures, and poetic texts" on their albums Rubber Soul and Revolver "encouraged a legion of young bands that were to create progressive rock in the early 1970s". Academics Paul Hegarty and Martin Halliwell identify the Beatles "not merely as precursors of prog but as essential developments of progressiveness in its early days". After the release of Rubber Soul, many "baroque-rock" works would soon appear, particularly due to its track "In My Life".  Citing a quantitative study of tempos in music from the era, musicologist Walter Everett identifies Rubber Soul as a work that was "made more to be thought about than danced to", and an album that "began a far-reaching trend" in its slowing-down of the tempos typically used in pop and rock music. Although the Kinks, the Yardbirds and the Beatles themselves (with "Ticket To Ride") had incorporated droning guitars to mimic the qualities of the sitar, Rubber Soul's "Norwegian Wood (This Bird Has Flown)" is generally credited as sparking a musical craze for the sound of the instrument in the mid-1960s -- a trend which would later be associated with the growth of raga rock, Indian rock, and the essence of psychedelic rock. In terms of bridging the relationship between music and hallucinogens, the Beatles and the Beach Boys were the most pivotal. Revolver ensured that psychedelic pop emerged from its underground roots and into the mainstream.  Author Carys Wyn Jones locates Sgt. Pepper's, along with Pet Sounds, to the beginning of art rock. Both albums are largely viewed as beginnings in the progressive rock genre due to their lyrical unity, extended structure, complexity, eclecticism, experimentalism and influences derived from classical music forms. For several years following Sgt. Pepper's release, straightforward rock and roll was supplanted by a growing interest in extended form. Several of the English psychedelic bands who followed in the wake of the Beatles' Sgt. Pepper's developed characteristics of the Beatles' music (specifically their classical influence) further than either the Beatles or contemporaneous West Coast psychedelic bands. AllMusic states that the first wave of art rock musicians were inspired by Sgt. Pepper's and believed that for rock music to grow artistically, they should incorporate elements of European and classical music to the genre.

Answer this question "What is progressiveness?" by extracting the answer from the text above.
Progressive rock (or art rock) grew out of the classically-minded strains of British psychedelia.