Backwards Time Travel and Backwards Causation

Abstract

Is it possible to travel back in time? Objections against backwards time travel frequently focus on the impossibility of changing the past. However, there is a deeper problem for backwards time travel. It seems that if backwards time travel were possible, the time traveler could causally affect the past, even if she could not alter the past. Is it possible to causally affect the past? David Lewis famously argues that backwards causation is possible as long as we understand causal dependence in terms of counterfactual dependence rather than temporal precedence. In this paper, I argue that this Lewisian line of reasoning is untenable. Lewis’ counterfactual theory of causation faces the so-called ‘problem of effects’. To solve this problem, he either has to reintroduce temporal precedence back into the definition of causation or stipulate that the closest possible world needs to be historically the same as the actual one. Either way, the possibility of backwards causation is immediately ruled out.