As with many van Vogt works, the novel uses a psychological element, in this case Intelligence Quotient or IQ. The novel shows various characters who are at different levels of IQ and as part of the action shows how differences in IQ result in differences in perception and attention as well as differences in being able to grasp a situation and make decisions.
The novel, like many of van Vogt's novels, has a hyper-rational tone in which many of the characters, especially those from the galactic civilization have little emotional depth. The description of the galactic civilization is one of a kind of caste system in which social roles are assumed based on one's level of IQ. The galactic civilization has set up an outpost on the planet Earth manned by a scientist and his daughter whose IQ are much higher than human however as the novel develops, other groups within the galactic civilization emerge who are much more intelligent.
The novel is about one group within the galactic civilization, the Dreegh, who are similar to vampires in that they must absorb the life energies of other beings in order to survive due to an accident. A space ship with a Dreegh couple arrives on Earth as scouts to determine if what is left of the Dreegh race can take over the Earth as a base and place to live thereby hiding from the galactic civilization.
In the novel van Vogt attempts to provide an idea as to the perceptual differences between beings of different IQ as well as differences in integration between mind and body with the ability to evaluate and assess a situation quickly and accurately, make decisions quickly, and act on those decisions quickly.
- "Astounding May 1942", https://archive.org/details/Astounding_v29n03_1942-05