The inhabitants of Roger Zelazny's Sign of the Unicorn are not human beings at all but a race of aristocrats who live in another dimension and who can slip in and out of Earth, which they call Shadow, when they feel like it. They are a squabbling crew of brothers and sisters, a sort of outsize Borgia family complete with daggers and swords and Renaissance castles, and the plot of the novel is closer to that of a thriller than to most science fiction plots. I mention the book, though, because it is good, because it solves the stylistic problem of science fiction in an interesting way, and because it represents a sort of boundary of science fiction, the place where it ends.

To take the last point first, the members of the family can speak to each other by means of decks of playing cards, and can...