We narrowly missed such a fate. There was a moment in time when a youthful Isaac faced a critical career choice: go on as a researcher or plunge full-time into writing. He chose writing and the world is extremely happy with the result.

Knowing that science fiction, in those primeval days, could not support a wife and family, Isaac chose to write about science fact and to make that his career, rather than biomedical research.

But suppose he had not?

Suppose, faced with that career choice, Isaac had opted for the steady, if unspectacular, career of a medium-level research scientist who wrote occasional science fiction stories as a hobby.

We would still have the substantial oeuvre of his science fiction tales that this anthology celebrates. We would still have “Nightfall” and “The Ugly Little Boy,” the original Foundation trilogy and novels such as Pebble in the Sky. We would, to return to the metaphor we started with, still have Isaac’s “metallic” output.

But we would not have his hydrogen and helium, the huge number of books that are nonfiction, mainly books about science, although there are some marvelous histories, annotations of various works of literature, and lecherous limericks in there, too.

If Isaac had toiled away his years as a full-time biomedical researcher and part-time science fiction writer, we would never have seen all those marvelous science books. Probably a full generation of scientists would have chosen other careers, because they would never have been turned on to science by the books that Isaac did not write. Progress in all fields of the physical sciences would have slowed, perhaps disastrously.

Millions of people allover the world would have been denied the pleasure of learning that they could understand the principles of physics, mathematics, astronomy, geology, chemistry, the workings of the human body, the intricacies of the human brain-because the books from which they learned and received such pleasures would never have been written.