Elijah Baley, the Earth-born detective, realizes that while the robots are the witnesses, the humans are the variables. He understands that Spacers—humans born on wealthy, low-population outer worlds—have a vastly different psychological makeup than Earthmen. They are intensely private, status-conscious, and often physically fragile.

The story also touches on the theme of identity and the concept of self. If there exists a mirror image of ourselves in another universe, then which one is the "real" us? Are we just a copy or an original?

(Note: The core of Asimov's solution usually hinges on the logical weight of the statements. Sabat’s robot claims an interaction occurred; Humboldt’s claims isolation. It is logically easier for the thief (Humboldt) to order his robot to hide an interaction than for Sabat to invent one.)