A traveler takes a book to the past, where it is copied, and the copy is the book he later receives. Every copy accounted for, the content written by no one.
Documented
Legal in the equations, nauseating to the intuition
The paradox is documented across philosophy of time travel: closed causal loops are permitted by general relativity's own solutions, and nothing in the mathematics forbids self-consistent information loops. Heinlein's By His Bootstraps (1941) supplied the name. The puzzle is not contradiction but provenance: conservation laws forbid objects from looping forever without wear, and information with no source violates every instinct about authorship, yet violates no equation.
Assessment: Documented: the stories, the physics' tolerance, the philosophical literature. Not documented: any actual loop. The paradox is a standing demonstration that self-consistent and explicable are different properties.
Stories
Beethoven's manuscript, the Terminator's chip
Fiction adores the loop: the traveler who hands Beethoven his own scores, Skynet reverse-engineered from its own wreckage, the watch in Somewhere in Time passed between lovers with no purchase date. The folk takeaway is wonder; the physicist's takeaway is that the audience never asks who wrote the notes.
Assessment: The examples are documented fiction. They dramatize the provenance problem perfectly and resolve it never, which is honest of them.
Interesting claims
The universe as its own bootstrap
The interesting claim escalates the loop to cosmology: perhaps the universe's contents, laws, or even existence loop without external cause, reality as the ultimate unwritten book. Some invoke it for ancient mysteries, the knowledge that appears in history with no visible development, anachronistic precision as the fingerprint of a loop.
Assessment: The cosmological version is unfalsifiable poetry. The archaeological version has a documented answer wherever checked: every claimed out-of-place artifact has either context or fraud behind it. The loop remains a legal fiction of physics, filed nowhere in evidence.
Sources and further reading
Heinlein, By His Bootstraps (1941), the name
Nahin, Time Machines (1999), the provenance problem analyzed
Thorne, Black Holes and Time Warps (1994), self-consistent loops in relativity
Deutsch and Lockwood, The Quantum Physics of Time Travel, Scientific American (1994)
Lewis, The Paradoxes of Time Travel (1976), the philosophical foundation
Worth asking
If information can exist without origin, what else in your world arrived from nowhere and simply loops?