A shadow thrown onto a wall is not photography. But if the wall is photosensitive and the shadow remains after the body has moved on, that is photography. Human creativity, since the beginning of art, has found ways to double the visible world. What photography did was to give the world a way to double its own appearance: the photograph results directly from what is, from the light that travels from a body through an aperture onto a surface.

But when the photograph outlives the body—when people die, scenes change, trees grow or are chopped down—it becomes a memorial. And when the thing photographed is a work of art or architecture that has been destroyed, this effect is amplified even further. A painting, sculpture, or temple, as a record of both human skill and emotion, is already a site of memory; when its only remaining trace is a photograph, that photograph becomes a memorial to a memory. Such a photograph is shadowed by its vanished ancestor.

— Teju Cole, Known and Strange Things, Faber & Faber, Ch. Memories of Things Unseen, 2016