While in London without his bandmates, McCartney took the hallucinogenic drug LSD (or "acid") for the first time, having long resisted Lennon and Harrison's insistence that he join them and Starr in experiencing its perception-heightening effects. According to author Jonathan Gould, this initiation into LSD afforded McCartney the "expansive new sense of possibility" that defined the group's next project, "Sgt. Pepper's Lonely Hearts Club Band". Gould adds that McCartney's succumbing to peer pressure allowed Lennon "to play the role of psychedelic guide" to his songwriting partner, thereby facilitating a closer collaboration between the two than had been evident since early in the Beatles' career. For his part, Lennon had turned deeply introspective during the filming of "How I Won the War" in southern Spain in September 1966. His anxiety over his and the Beatles' future was reflected in "Strawberry Fields Forever", a song that provided the initial theme, regarding a Liverpool childhood, of the new album. On his return to London, Lennon embraced the city's arts culture, of which McCartney was a part, and shared his bandmate's interest in avant-garde and electronic-music composers such as Karlheinz Stockhausen, John Cage and Luciano Berio.