Science Fiction
Hold on Shmoopers, we're about to get all professor-y and go full intellectual on everyone's favorite genre: Science Fiction. Please don't cry. It will be okay. We promise.
More Than Human speculates about evolution and progress. The novel ponders what might happen if evolution headed in a mental direction, rather than a physical one. It considers both thoughts and passions to be products of the mind—you know, mental stuff. So the characters get to have cool mind-powers like teleportation and telekinesis, and the book shows where all their passionate mind-merging and thinking might lead: to an advanced, ethical society.
Now, this book differs from many other sci-fi books in that it mostly focuses on the "soft" science of psychology rather than "hard" sciences like chemistry or physics. That places Theodore Sturgeon closer to authors such as Ursula K. Le Guin, who emphasized the soft science of anthropology, rather than, say, Arthur C. Clarke, who took pains to accurately depict the nitty-gritty details of hard sciences.
Sturgeon was more interested in making philosophical—you might even say spiritual—points than in getting his science facts correct. He once praised the genre for giving him the freedom to ask unusual questions.
It's worth pointing out that many definitions of science fiction require works to be based on strict science. Today, people might consider "psionic" mind powers more the stuff of fantasy fiction than of science fiction—which is a major difference, apparently. But the novel was written in 1953, when influential sci-fi editors, such as John W. Campbell, had shaped the genre such that speculation about psionics was considered legitimate scientific musing. We're not complaining.