John Lennon and Paul McCartney visited Indica Books & Gallery, which had opened in March 1966 at 6 Masons Yard, London. Lennon was looking for a copy of The Portable Nietzsche, but emerged with something quite different.
The shop and gallery focused on the contemporary underground literary and art scene, and was owned by Barry Miles, Peter Asher and John Dunbar. McCartney had invested £5,000 to help open the venture, and had designed the bookshop's wrapping paper and assisted with decorating the interior.
During this visit Lennon bought a copy of The Psychedelic Experience: A Manual Based On The Tibetan Book Of The Dead by Timothy Leary, Richard Alpert, and Ralph Metzner. At the beginning of the book's introduction he found a line which would be adapted for Tomorrow Never Knows: "When in doubt, relax, turn off your mind, float downstream."
Leary was the one going round saying, take it, take it, take it. And we followed his instructions in his 'how to take a trip' book. I did it just like he said in the book, and then I wrote Tomorrow Never Knows, which was almost the first acid song: 'Lay down all thoughts surrender to the void,' and all that shit which Leary had pinched from The Book Of The Dead.
Upon returning to his home in Weybrige Lennon recorded himself reciting Leary's words, which he played back during a subsequent LSD trip. The Beatles began recording Tomorrow Never Knows on 6 April 1966, just five days later the visit to Indica.