"to behold an Island in the Air"
Gulliver discovers Laputa
Public DomainGulliver discovers Laputa

Isaac Asimov in The Annotated Gulliver's Travels remarks:

Others had invented fanciful flying cities before this, but Jonathan Swift was the first to attempt an explanation of its workings in line with the findings of contemporary science. This section of the book is therefore true science fiction, perhaps the earliest example we have of it. To include earlier works as science fiction involves broadening the definition of the genre to include works not strictly based on a scientific background, and thus little more than adventure fantasy -- like, for instance, the first two parts of Gulliver's Travels.

Satirically, Laputa is supposed to symbolize the court of Great Britain which towered high above the rest of the kingdom in terms of power and social position.

Swift also pokes fun at the world of science which involves itself in abstract thought high above the interests of lowly, ordinary human beings.