"But why do I simply shrug and stop reading if a whodunit turns out to be weary and derivative, while I feel acutely embittered when I find the same qualities in s-f? I see now that it is because s-f is a form which, more than almost any other, by its very nature demands creative originality. The detective story and even the more modern psychological crime novel are—like the western, the love story, the historical romance—fixed forms, in which the creative challenge lies largely in seeing what the author can do within established boundaries. S-f is—or perhaps better, should and must be a literature of stimulus and fresh horizons."