“[Causal loops are] closed causal chains in which some of thecausal links are normal in direction and others are reversed ....Each event on the loop has a causal explanation, being caused byevents elsewhere on the loop. This is not to say that the loop as awhole is caused or explicable. It may not be.”• “A nice example of a causal loop involves the transfer ofinformation. Imagine a time traveller who goes back in time afew years and talks to his earlier self. They discuss time travel,and:in the course of the conversation his older self told his youngerself how to build a time machine…”