Here's what Wikipedia says about Rubber Soul - my boldface:
In mid-October 1965, the Beatles entered the recording studio; for the first time when making an album, they had an extended period without other major commitments. Released in December, Rubber Soul has been 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. Biographers Peter Brown and Steven Gaines attribute the new musical direction to "the Beatles' now habitual use of marijuana", an assertion confirmed by the band – Lennon referred to it 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 the world of 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 their lyrics grew more artful, fans began to study them for deeper meaning. Of "Norwegian Wood" Lennon commented: "I was trying to be sophisticated in writing about an affair ... but in such a smokescreen way that you couldn't tell."
While many of Rubber Soul 's more notable songs were the product of Lennon and McCartney's collaborative songwriting, it also featured distinct compositions from each, though they continued to share official credit. The song "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".