 Suppose that Tim learns that his grandfather had done something terrible in the 1920s, several years before the birth of Tim’s mother. Suppose also that Tim has invented a time machine. While it may seem that Tim could go back in time and kill his grandfather to prevent him from doing this terrible thing, in fact, it is impossible for Tim to kill his grandfather. The past has already happened. It cannot be changed. Since Tim’s grandparents had Tim’s mother, who went on to have Tim, it must be the case that Tim did not kill his grandfather.
 Assume that Tim discovers that his granddad had accomplished something ghastly in the 1920s, quite a long while before the introduction of Tim's mom. Assume additionally that Tim has concocted a time