The four stories are loosely related as speculation on what—that we are not already aware of—makes one a human. "Jerry Is a Man" makes the most straightforward examination of the theme, a court making a legal ruling on the human rights of genetically engineered intelligent creatures. "Gulf", a story connected by its story materials to Kuttner's Baldy stories and Wilmar Shiras' In Hiding, suggests that superior individuals already living among us might become a new step in hominid evolution; "Lost Legacy" suggests that every person has unused paranormal abilities that can be awakened by esoteric training comparable to that used by the supermen of "Gulf". "Elsewhen" suggests that the human mind is not bound to our here-and-now "slum of space-time" but can go voyaging into alternative timetracks of possibility. Although written in 1939, using materials popularized by J. W. Dunne, the story has gained new currency in the wake of the Wheeler-Everett "Many Worlds" hypothesis.
The story materials of all four novellas were revisited by Heinlein in later, more expansive novels. A number of figures of "Lost Legacy", for example, are carried into Stranger in a Strange Land (1961), a book which ultimately could be said to have the same theme as the 1939 novella. Situations and individuals from both "Gulf" and "Jerry Was a Man" are examined in Friday (1982). And the "multiverse" concept first explored in "Elsewhen" gets very full treatment in Heinlein's last novels, particularly The Number of the Beast (1980), The Cat Who Walks Through Walls (1985) and To Sail Beyond the Sunset (1987).