Ted Danson

If you take one rivet out of an airplane, it will be all right, it'll keep flying. You take another rivet out of the airplane and it still flies. So what the heck, let's take more rivets out of the airplane, and sooner or later, the airplane drops from the sky.

The historical novelist has to consider what has actually happened, while the SF writer is dealing in possibilities, but they are both in the business of imagining a world unlike our own and yet connected to it.