Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least   problematic.[18]  The classic argument against backwards causation is the bilking  argument.[19]  Faced with the claim that some event A causes an earlier event B, the proponent of the bilking objection recommends an attempt to decorrelate A and B—that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A, and A often occurs without B occurring, and so A cannot be the cause of B. If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A, rather than vice versa.