There are a number of references, some oblique and some more direct. One of the clearer you've probably not hit yet (at least at the writing of your comment).

There are a number of references, some oblique and some more direct. One of the clearer you've probably not hit yet (at least at the writing of your comment). But you've earlier passed, for instance, "But the idea of living out his life in the mind-groping-ahead-through-possible-futures that guided hurtling spaceships appalled him. It was a way, though. And in meeting the possible future that contained Guildsmen he recognized his own strangeness." > They certainly haven't been an important part of the story. Directly? No. But they're why the spice is necessary to space travel, which plays a major role in the economics (and thereby politics) of the situation.