my mind was young and the barrier between reality and make-belief had not yet hardened into the shell that cocoons us in adult life. The barrier was soft, pliable and for a moment, thanks to the kindness of a stranger and the power of a good storytelling voice, I made the short journey — and returned.Let's put romanticism about childhood experience aside (Fforde exploits this cleverly in this passage, but it's not this rhetorical aspect I'm interested in here). What is the function of such explanations about the "barrier between reality and make-belief"? (Which are explanations, really, about the theory or metaphysics underlying the world of the book.) What do they help to achieve in the process of our consuming (or appreciating) fiction?
It is sometimes said that their function is to motivate what is going on in this instance of fiction (i.e. in this book or movie), that they are included in order to make the goings-on (the events, the reaction of the characters) plausible. Their function then, on this view, is to help us believe what we observe; they move us from incredulity to acceptance; they enable suspension of disbelief.
Now this is certainly not entirely wrong: such formulations probably do all these things. But suggestive though it may be, this way of putting it also obscures an important distinction. We don't perceive worlds of fiction. We imagine them. (No doubt we perceive, visually and auditorily, what goes on on a movie screen. But that's not the same as perceiving the imaginary world. In order for the latter to become accessible, there must be a process of imagination, just as there must be such a process when we read a novel. The imagination may be greatly supported by the movie images and sounds, both of which aren't there when we read prose. But what constitutes the fictional world, in both cases, is a process of imagination.) In contrast to perception, then, what we do isn't in the first instance belief-forming, but something more like stipulation.
What phrases such as this one do is not to make the fictional world believable, I'm going to contend. Rather, these are hints to the imagination, design hints. They guide the imagination in fundamental aspects of the fictional world it constructs. They control the frame of what we imagine.
For instance, in the quote above from the Fforde novel: what we are told here is not that, contrary to our everyday belief, the borders between reality and fiction might be more porous than we thought, after all. (How would that be a plausible claim, even if it were made with the intention to appear as one? Countless experiences and the whole body of common knowledge weigh in favor of the contrary.) Instead, it is an indicator, given by the author, of the kind of fictional world we're in. It gently nudges our imagination in a certain direction. We're to imagine a fictional world (that is the world of the novel The Eyre Affair) in which, much in contrast to the real world, the borders between this world itself and any nested fiction (nested unreality, such as that of the book Jane Eyre as referred to in the novel The Eyre Affair) are permeable. It's more subtle than the traditional "Imagine, dear reader, a world in which the borders between reality and fiction can be bent, so that one might travel between the those two..." — but the function is exactly the same. It isn't intended to make such a thought more plausible or believable; it's intended to point out ways for us to imagine such a world.
Such pointers, such hints at differences between the fictional world we're dealing with and the real world, have something in common with the bits of fictional export I mentioned in the previous post. The author provides us with them so that we are better able to imagine the fictional world in question. The materials for fictional export hold also in the real world, and they are included by the author in his fictional world because they are required for the narrative. (Think explanations of forensic methods in crime fiction.) The differential hints I'm discussing here are of course explanations of differences between reality and fiction (this particular fiction). But they serve the same function: helping us to better understand what sort of world we are to imagine in order to make sense of the narrative.
 Jasper Fforde, The Eyre Affair. London: Hodder and Stoughton 2001, 63.
 Both perception and imagination are incredibly complicated processes, which haven't been researched in all detail by cognitive science and other disciplines yet. A good starting point for reading up about the differences in phenomenology is Colin McGinn's Mindsight. Image, Dream, Meaning, Cambridge, MA: Harvard University Press 2004, especially chapter 1.