H. G. Wells's The Time Machine (1895) set the standard for time travel stories. Since the publication of The Time Machine, authors have sought to define the problems that would be associated with time travel. For instance, there are the time "paradoxes," such as how could a character be born if he goes back in time and murders his grandmother before his mother is conceived — if he is not born then his mother is born and then he is born even though he cannot be born. Van Vogt answers this with multiple time lines that allow a character to be born in one line and his grandmother to be murdered in another. Another important "paradox" is the problem of how a character can travel back in time and exist simultaneously with himself. In Quest for the Future, such a character merges with himself, eventually becoming a...