The ideas in Slan continued to interest van Vogt throughout his career. In The World of Null-A (1948; also published as The World of A) and its sequels The Players of Null-A (1956; also published as The Players of A, The Pawns of Null-A, and The Pawns of A) and Null-A Three (1985), van Vogt explored the social implications of a superior way of thinking. Instead of emphasizing biological evolution as he did in Slan, he emphasized a social evolution brought on by some human beings learning to use their brains better than other human beings are able to.

In van Vogt's view, this superior way of thinking would result in people prepared to logically respond to any situation, no matter how unfamiliar. This is taken to an extreme in the protagonist, who is killed but each time awakens in a new body in a new place, often a...