This article is from the Isaac Asimov FAQ, by Edward J. Seiler email@example.com and John H. Jenkins firstname.lastname@example.org with numerous contributions by others.
The whole Foundation series is rife with contradictions. There are two
main reasons for this.
First of all, Asimov simply didn't enjoy sweating over details in his
fiction. There are a number of things Asimov enjoyed about writing --
that's why he wrote so much -- but purging his fiction of contradictions
was not one of them. As early as 1945, he was finding it more effort than
it was worth to keep up consistency in the Foundation stories and tried
(three times) to end the series so that he wouldn't have to deal with it.
Secondly, Asimov's overall plan for the series changed. For example, the
robot stories and Foundation stories were originally conceived as existing
in separate fictional universes. It wasn't until the 1980s that he
started to tie them together explicitly. Other examples would involve
major spoilers for some of the later books.
(Also, the stories were written over the course of fifty years, starting
from a time when Asimov was at the unspectacular beginning of his career
and the Golden Age was a year old, to a time when Asimov was one of
science fiction's Big Three and John Campbell, for whom the earliest
stories were written, dead for twenty years. It should not be surprising
that the seventy-year-old Grand Master should find some of the ideas of
the twenty-year-old apprentice not quite up to snuff and not worth