From the Introductory:

JOHN W. CAMPBELL first started writing in 1930 when his first short story, When the Atoms Failed, was accepted by a science-fiction magazine. At that time he was twenty years old and still a student at college. As the title of the story indicates, he was even at that time occupied with the significance of atomic energy and nuclear physics.

For the next seven years, Campbell, bolstered by a scientific background that ran from childhood experiments, to study at Duke University and the Massachusetts Institute of Technology, wrote and sold science-fiction, achieving for himself an enviable reputation in the field.

In 1937 he became the editor of Astounding Stories magazine and applied himself at once to the task of bettering the magazine and the field of s-f writing in general. His influence on science-fiction since then cannot be underestimated. Today he still remains as the editor of that magazine's evolved and redesigned successor, Analog.

From the INTRODUCTION:

These stories were written nearly a quarter of a century ago, for the old Amazing Stories magazine. The essence of any magazine is not its name, but its philosophy, its purpose. That old Amazing Stories is long since gone; the magazine of the same name today is as different as the times today are different from the world of 1930.

Science-fiction was new, in 1930; atomic energy was a dream we believed in, and space-travel was something we tried to understand better. Today, science-fiction has become a broad field, atomic energyùdespite the feelings of many present adults!ùis no dream. (Nor is it a nightmare; it is simply a fact, and calling it a nightmare is another form of effort to push it out of reality.)

In 1930, the only audience for science-fiction was among those who were still young enough in spirit to be willing to hope and speculate on a new and wider futureùand in 1930 that meant almost nothing but teen-agers. It meant the brightest group of teen-agers, youngsters who were willing to play with ideas and understandings of physics and chemistry and astronomy that most of their contemporaries considered ôtoo hard work.ö

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I grew up with that group; the stories I wrote over the years, and, later, the stories I bought for Astounding Science Fiction changed and grew more mature too. Astounding Science Fiction today has many of the audience that read those early stories; they're not high school and college students any more, of course, but professional engineers, technologists and researchers now. Naturally, for them we need a totally different kind of story. In growing with them, I and my work had to lose much of the enthusiastic scope that went with the earlier science fiction.

When a young man goes to college, he is apt to say, ôI want to be a scientist,ö or ôI want to be an engineer,ö but his concepts are broad and generalized. Most major technical schools, well knowing this, have the first year course for all students the same. Only in the second and subsequent years does specialization start.