Backwards causation• “Time travel into the past necessarily involves backwardscausation (with respect to external time)…Is it an objection to thevery possibility of travel into the past that it requires backwardscausation? Only if the idea of backwards causation, the idea of aneffect preceding its cause, is incoherent…[N]one of the leadingtheories of causation rules out the possibility of backwardscausation. Hence it is not reasonable to object to travel into thepast simply on the grounds that it inevitably involves backwardscausation. Indeed, if travel into the past is not ruled out by otherconsiderations, we could appeal to the possibility of time travel asan argument for the possibility of backwards causation.”