In this paper I wish to examine the claim that it would be possible for us now to do something which would be the posterior efficient cause of some past event. I am not prepared to discuss the physics of elementary particles, and I will not consider what is sometimes called time reversal. Rather my analysis will be limited to cases in which it is alleged that we, in a world of middle-sized physical objects where most causes precede or are (...) simultaneous with their effects, could conceivably do something so that something else should have happened. I will argue that some of the cases which meet this description are indeed backwards causation if one is prepared to make certain assumptions about time. I will not evaluate these assumptions; rather I will try to clarify them and to make plain their implications for causality. For the argument about backwards causation is most fundamentally, or so I will try to show, an argument about the nature of time. (shrink)