Once committed to his imaginary kingdom, the writer is not a monarch but a subject. Characters must appear plausible in their own setting, and the writer must go along with their inner logic, Happenings should have logical implications. Details should be tested for consistency. Shall animals speak? If so, do all animals speak? If not, then which — and how? Above all, why? Is it essential to the story, or lamely cute? Are there enchantments? How powerful? If an enchanter can perform such-and-such, can he not also do so-and-so?
True enough, the writer of fantasy can start with whatever premises he chooses (actually, the uncomplicated ones work best). In the algebra of fantasy, A times B doesn’t have to equal B times A. But, once established, the equation must hold throughout the story. You may set your own ground rules and, in the beginning, decree as many laws as you like — though in practice the fewer departures from the “real” world the better. A not-very-serious breach and the fantasy world explodes just as surely as if a very real hydrogen bomb had been dropped on it. With inconsistency (so usual in the real world), the machinery moving the tale grinds and screeches; the characters cease to be imaginary and become simply unreal. Truth drains out of them. Admittedly, certain questions have to be begged, such as “How did all these people get here in the first place?” But they are like the axioms of geometry, questioned only by metaphysicians.
Melancholy men, they say, are the most incisive humorists; by the same token, writers of fantasy must be, within their own frame of work, hardheaded realists. What appears gossamer is, underneath, solid as prestressed concrete. What seems so free in fantasy is often inventiveness of detail rather than complicated substructure. Elaboration — not improvisation.
Perhaps the problem is that the books are novels rather than short stories. In a story, you can fire an outrageous proposition into the air – car crashes are sexy! – and then scarper while the audience is still dazzled by the bright explosive audacity of it, whereas a novel gives readers time to poke around in the undergrowth and catch you fiddling away trying to light the next fuse.
He has said elsewhere that his stories aren’t set in the future so much as in ‘a kind of visionary present’. This insight – that good science fiction is an urgent way of writing about the present – shouldn’t need repeating, though there’s a lingering literary prejudice which means that it does. Another way of putting it would be to say that good science fiction tells a number of particular untruths in order to reveal a more general truth, which is to say that it is in essence no different from any other kind of good fiction.
The Major had an answer of this kind to most of Pen’s objections, and Pen accepted his uncle’s replies, not so much because he believed them, but because he wished to believe them. We do a thing—which of us has not?—not because “everybody does it,” but because we like it; and our acquiescence, alas! proves not that everybody is right, but that we and the rest of the world are poor creatures alike.
William Makepeace Thackeray, The History of Pendennis.
We alter very little. When we talk of this man or that woman being no longer the same person whom we remember in youth, and remark (of course to deplore) changes in our friends, we don’t, perhaps, calculate that circumstance only brings out the latent defect or quality, and does not create it. The selfish languor and indifference of to-day’s possession is the consequence of the selfish ardour of yesterday’s pursuit: the scorn and weariness which cries vanitas vanitatum is but the lassitude of the sick appetite palled with pleasure: the insolence of the successful parvenu is only the necessary continuance of the career of the needy struggler: our mental changes are like our grey hairs or our wrinkles—but the fulfilment of the plan of mortal growth and decay: that which is snow-white now was glossy black once; that which is sluggish obesity to-day was boisterous rosy health a few years back; that calm weariness, benevolent, resigned, and disappointed, was ambition, fierce and violent, but a few years since, and has only settled into submissive repose after many a battle and defeat.
William Makepeace Thackeray, The History of Pendennis.

After the first season, indeed, Lady Rockminster, who had taken up Lady Clavering, put her down; the great ladies would not take their daughters to her parties; the young men who attended them behaved with the most odious freedom and scornful familiarity; and poor Lady Clavering herself avowed that she was obliged to take what she called ‘the canal’ into her parlour, because the tip-tops wouldn’t come.”

She had not the slightest ill-will towards “the canal,” the poor dear lady, or any pride about herself, or idea, that she was better than her neighbour; but she had taken implicitly the orders which on her entry into the world her social godmother had given her: she had been willing to know whom they knew, and ask whom they asked. The “canal,” in fact, was much pleasanter than what is called “society;” but, as we said before, that to leave a mistress is easy, while, on the contrary, to be left by her is cruel: so you may give up society without any great pang, or anything but a sensation of relief at the parting; but severe are the mortifications and pains you have if society gives up you.

William Makepeace Thackeray, The History of Pendennis.
O Clarissas of this life, O you poor little ignorant vain foolish maidens! if you did but know the way in which the Lovelaces speak of you: if you could but hear Jack talking to Tom across the coffee-room of a Club; or see Ned taking your poor little letters out of his cigar-case, and handing them over to Charley, and Billy, and Harry across the messroom table, you would not be so eager to write, or so ready to listen! There’s a sort of crime which is not complete unless the lucky rogue boasts of it afterwards; and the man who betrays your honour in the first place, is pretty sure, remember that, to betray your secret too.
William Makepeace Thackeray, The History of Pendennis.
What could Fanny expect when suddenly brought up for sentence before a couple of such judges? Nothing but swift condemnation, awful punishment, merciless dismissal! Women are cruel critics in cases such as that in which poor Fanny was implicated; and we like them to be so; for, besides the guard which a man places round his own harem, and the defences which a woman has in her heart, her faith, and honour, hasn’t she all her own friends of her own sex to keep watch that she does not go astray, and to tear her to pieces if she is found erring?
William Makepeace Thackeray, The History of Pendennis.