The Bootstrap Paradox

A related puzzle is the bootstrap paradox, also called a causal loop. You travel to the past and give Shakespeare his plays before he writes them. He copies them. The plays exist, but nobody actually wrote them — they bootstrapped themselves into existence through a loop with no origin.

This challenges the principle that every effect must have a cause. In a causal loop, the effect causes the cause that causes the effect. Information appears from nowhere, with no point at which it was ever created.

The grandfather paradox creates an outright contradiction; the bootstrap paradox breaks no rule of logic, yet leaves you with knowledge that no one ever thought up.