  One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible.[21] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question.[22]  Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops.[23] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples.