 The title derives from the colloquialism "plastic soul" and was the Beatles' way of acknowledging their lack of authenticity compared to the African-American soul artists they admired.After "A Hard Day's Night" in 1964, it was the second Beatles LP to contain only original material.The songs demonstrate the Beatles' increasing maturity as lyricists and, in their incorporation of brighter guitar tones and new instrumentation such as sitar, harmonium and fuzz bass, the group striving for more expressive sounds and arrangements for their music.