But if I was arguing in Trust me, I'm telling stories for being allowed to play fast and loose with historical (or, indeed, any other) facts, I do see that there's not a lot of point in fiction that doesn't grapple with the realities of human existence in some way. It doesn't need Bruno Bettelheim to tell us that even fairy stories say important things to us: that enchantment has its uses. A fairy story may also be - pace Freud - a growing-up myth; many an opera - pace Jung - is an integration of animus and anima; and a well-crafted modern comedy makes us laugh at our uneasily comfortable modern lives which we never thought to laugh about. And that's not to exclude the science fiction and fantasy branches of the fiction tree either: it's simply that they play by different set of rules about what's defined as 'believable' - dragons, for instance.
So even though the definition of a novelist's trade could be that We Make Things Up, any novel needs a bedrock of human existence - human truth, if not historical or geographical fact - because without it fiction is pointless. The difficulty is that every reader has a very slightly different frame of reference for testing such truth, based on a slightly different experience of the world. Write about what you can make me believe you know is my slogan for aspiring writers, but what I'll believe as a reader is as much about me as it is about you. I never did discover what it was about Anna, in The Mathematics of Love, that meant a fellow-workshopper didn't believe she'd been brought up on a council estate, but since the others in the workshop did believe she had, I didn't feel obliged to re-write Anna, or her childhood.
That ought to mean that the further away the world of a novel is from the world of its readers, the faster and looser I can play with mere facts, without shaking my readers' faith in that bedrock of human truth, because who can tell me I'm 'wrong'? But, lacking the believability so easily established (in theory) by the novelist of modern life with an Ordnance Survey map and a bit of eavesdropping on buses, you can argue that those of us who set our fiction in other worlds - the past, the future, different continents or different galaxies - have to drill even deeper into human bedrock before we can start to build our story.