Page "Foundation series" Paragraph 33

In 1982, Asimov gave in after a thirty-year hiatus, and wrote what was at the time a fourth volume: Foundation's Edge.

This was followed shortly thereafter by Foundation and Earth.

The story of this volume ( which takes place some 500 years after Seldon ) ties up all the loose ends, but opens a brand new line of thought in the last dozen pages.

According to his widow Janet Asimov ( in her biography of Isaac, It's Been a Good Life ), he had no idea how to continue after Foundation and Earth, so he started writing the prequels.