Slan focuses on two main themes.

The first is the question of what makes a person fully human. The second is the problem of truth in a world in which there is far more that is knowable than any single person could ever know. These are important questions.

One of the qualities that makes van Vogt's work stand out from most popular fiction is its working out challenging ideas in detail. This careful thinking through of complicated ideas unifies van Vogt's novels and gives his elaborate plots meaning beyond simple entertainment. This same working out of ideas has sometimes infuriated critics who disagree with them, as well as befuddling others who do not understand them.

When examining Slan's theme of what makes a person fully human, one can readily see why van Vogt's fiction is controversial and capable of arousing fierce passions in readers. Critics have failed to...