Pink Floyd made this album in the strangest possible position: on top, with nowhere to go. The Dark Side of the Moon had conquered the world, and the four men who made it walked into the next sessions hollowed out, unsure what the band was even for. So they made a record about absence. About the people and the parts of yourself that go missing while you are looking the other way.

The ghost is Syd Barrett, the founder they had lost years before. They were deep in “Shine On You Crazy Diamond,” the grieving suite that wraps around the whole record, when a heavy, shaven-headed stranger drifted into Abbey Road. No hair, no eyebrows, unrecognisable. It took them a while to understand they were watching Syd. Waters wept. They were mourning a man on tape who was standing in the room.

Even the cover hides itself: two businessmen shaking hands, one of them on fire, sealed in black shrink-wrap so you could not see the art until you owned it. Gilmour still calls it his favourite Floyd album. He is not alone.