an album, they had an extended period without other major commitments. Until this time, according to George Martin, "we had been making albums rather like a collection of singles. Now we were really beginning to think about albums as a bit of art on their own." Released in December, Rubber Soul was hailed by critics as a major step forward in the maturity and complexity of the band's music. Their thematic reach was beginning to expand as they embraced deeper aspects of romance and philosophy, a development that NEMS executive Peter Brown attributed to the band members' "now habitual use of marijuana". Lennon referred to Rubber Soul as "the pot album" and Starr said: "Grass was really influential in a lot of our changes, especially with the writers. And because they were writing different material, we were playing differently." After Help!s foray into classical music with flutes and strings, Harrison's introduction of a sitar on "Norwegian Wood (This Bird Has Flown)" marked a further progression outside the traditional boundaries of popular music. As the lyrics grew more artful, fans began to study them for deeper meaning. While some of Rubber Souls songs were the product of Lennon and McCartney's collaborative songwriting, the album also included distinct compositions from each, though they continued to share official credit. "In My Life", of which each later claimed lead authorship, is considered a highlight of the entire Lennon–McCartney catalogue. Harrison called Rubber Soul his "favourite album", and Starr referred to it as "the departure record". McCartney has said, "We'd had our cute period, and now it was time to expand." However, recording engineer Norman Smith later stated that the studio sessions revealed signs of growing conflict within the group – "the clash between John and Paul was becoming obvious", he wrote, and "as far as Paul was concerned, George could do no right". In 2003, Rolling Stone ranked Rubber Soul fifth among "The 500 Greatest Albums of All Time", and AllMusic's Richie Unterberger describes it as "one of the classic folk-rock records". Controversies, Revolver and final tour Capitol Records, from December 1963 when it began issuing Beatles recordings for the US market, exercised complete control over format, compiling distinct US albums from the band's recordings and issuing songs of their choosing as singles. In June 1966, the Capitol LP Yesterday and Today caused an uproar with its cover, which portrayed the grinning Beatles dressed