Science fiction is often described, and even defined, as extrapolative. The science fiction writer is supposed to take a trend or phenomenon of the here-and-now, purify and intensify it for dramatic effect, and extend it into the future. "If this goes on, this is what will happen." A prediction is made. Method and results much resemble those of a scientist who feeds large doses of a purified and concentrated food additive to mice, in order to predict what may happen to people who eat it in small quantities for a long time. The outcome seems almost inevitably to be cancer. So does the outcome of extrapolation. Strictly extrapolative works of science fiction generally arrive about where the Club of Rome arrives: somewhere between the gradual extinction of human liberty and the total extinction of terrestrial life.

This may explain why many people who do not read science fiction describe it as “escapist,” but when questioned further, admit they do not read it because “it's so depressing.”

Almost anything carried to its logical extreme becomes depressing, if not carcinogenic.

Fortunately, though extrapolation is an element in science fiction, it isn't the name of the game by any means. It is far too rationalist and simplistic to satisfy the imaginative mind, whether the writer's or the reader's. Variables are the spice of life.

This book is not extrapolative. If you like you can read it, and a lot of other science fiction, as a thought-experiment. Let's say (says Mary Shelley) that a young doctor creates a human being in his laboratory; let's say (says Philip K. Dick) that the Allies lost the second world war; let's say this or that is such and so, and see what happens… In a story so conceived, the moral complexity proper to the modern novel need not be sacrificed, nor is there any built-in dead end; thought and intuition can move freely within bounds set only by the terms of the experiment, which may be very large indeed.