As a form, science fiction conceals homogeneity beneath apparent diversity. The diversity can be seen by looking at the range of paperbacks in any bookshop. One finds lumped together ‘end of the world’ stories, galactic empire stories, stories of the near future and, via time travel, of the very far past, as well as stories that have nothing to do with science at all but depend on magic, or the fantasy type known as ‘sword and sorcery’. One might well think that the inclusion of all these under one heading is just a mistake, that the diversity is genuine. There are two reasons for thinking that is not so: that there is something holding all this diversity together. One is temporary and practical; the other is an element that regular readers recognise, something that forms a large part of the genre’s appeal....