Dune Messiah should be considered not as a sequel to the massive Dune novel, but as the penultimate section of an as-yet-incomplete work—whether trilogy, tetralogy or five-act epic matters not.

The objection to criticizing Dune Messiah as a simple sequel to the earlier novel is that almost invariably it has been seen as some sort of "repeat performance," and as such judged in terms of what the critic thought were the important themes of its predecessor. Considering it merely as a further installment, on the other hand, allows the critic the possibility that the important themes may only now be becoming apparent. (p. 16)

I suspect that in a large part the admiration Dune has commanded has come from the scope and the detail of its author's creation…. But the true scope of science fiction is not as an exhibition of the author's fertile and untrammeled imagination, but...