"Tomorrow Never Knows" is the final track of The Beatles' 1966 studio album Revolver but the first to be recorded. Credited as a Lennon/McCartney song, it was written primarily by John Lennon. Music critic Richie Unterberger of Allmusic said it was "the most experimental and psychedelic track on Revolver, in both its structure and production."
The song has a vocal put through a Leslie speaker cabinet (which was normally used as a loudspeaker for a Hammond organ) and uses automatic double tracking (ADT) to double the vocal image. Tape loops prepared by Paul McCartney were mixed in and out of the Indian-inspired modal backing underpinned by Ringo Starr's irregular drum pattern.
John Lennon wrote the song in January 1966, with lyrics adapted from the book The Psychedelic Experience: A Manual Based on the Tibetan Book of the Dead by Timothy Leary, Richard Alpert, and Ralph Metzner, which in turn was adapted from the Tibetan Book of the Dead. Although Peter Brown believed that Lennon's source for the lyric was the Tibetan Book of the Dead itself, which, he said, Lennon read whilst consuming LSD, George Harrison later stated that the idea for the lyrics came from Leary's, Alpert's and Metzner's book and McCartney confirmed this, stating that he and Lennon had visited the newly opened Indica bookshop — Lennon was looking for a copy of The Portable Nietzsche— and Lennon had found a copy of The Psychedelic Experience that contained the lines: "When in doubt, relax, turn off your mind, float downstream".
Lennon bought the book, went home, took LSD, and followed the instructions exactly as stated in the book. The book held that the "ego death" experienced under the influence of LSD and other psychedelic drugs is essentially similar to the dying process and requires similar guidance.