 The result in Gretta’s case generalises to the extent that causes are overdetermined by their effects but effects are not overdetermined by their causes. Overdetermination of effects by causes does of course happen – as when the victim is simultaneously shot by several assassins – but it is relatively rare, and even when it happens the effect is overdetermined only by a handful of events. By contrast, the leaving of traces is ubiquitous – and (or so Lewis needs to think) the extent of overdetermination, in any given case, is much greater than in cases of cause-to-effect overdetermination. Both of these, however, are contingent features of the actual world (or so Lewis claims; but see §2.1 below).