"In potatoes as a concept, yes. But do you really believe that you're a lucky enough person to find them fully cooked under rocks?" he asks reasonably. "Say it honestly, now, Scraggly-san: is that something the universe would really do for you?"
"Until you trip over something," he points out, still reasonably, "or run into a trap, or someone catches up just enough to hamstring you from behind."
Xellos gives Rincewind a you're giving me a headache - do you really think that's a good idea? look. "I know you're not fool enough to think that you'll be allowed to glue the plate to your head. Fantasizing quietly is all right. But you're a person with more self-preservation than to really think that's a choice you can make, aren't you."