Readers often complain that van Vogt's characters are poorly developed. For instance, in Slan the ruthless Joanna Hillory shifts from being Jommy Cross's implacably murderous enemy to wanting to be his wife, because, she says, he brought her hope by showing concern for the welfare of human beings. This shift of character is not credible, although it serves as a contrast late in Slan to Jommy Cross's change into a calculatedly ruthless manipulator of people.

Ideas dominate the narrative of Slan, and it is not surprising that characters seem under-developed and poorly motivated. Some like Joanna Hillory shift personalities as the requirements of the plot dictate. Her change of faith enables Jommy Cross to escape certain death. On the other hand, Kier Gray turns out to be a special kind of slan who is actually working for a better future primarily to give Slan a happy ending. That...