Fictional realism

Saw this from one of the critics...

"The secret of writing successful fantasy fiction is not to make all things indeterminate and possible, but to alter only a single, isolated aspect of reality."

I don't know who said this, cause the author did not quote it, but it's safe to say that H.G. Wells mentioned something similar in the preface of Seven Famous Novels, cause that's where the author said he got the idea from. Anyway, it continues ...

"In other words, to be believable in producing this kind of fantasy one may write a 'what-if-pigs-could-fly' kind of story, but one needs to limit it exclusively to flying pigs. Introducing flying human beings, for example, would be to break the rules."

Well, it's kinda interesting, though it may not necessary be true. I mean, if this rule applies for almost every fictional story, then it will be kinda boring, right? Though I do tend to believe that it would be a lot easier for someone not so experienced in writing to write a fiction that consists only of one altered reality (not that it'll be easy, but easier at least). Oh well, it's always easier to learn realism before working on fantasies and imaginations...

Oh and erm, for that flying pigs part, what if someone looks like a pig and has a figure of a pig...? Will he or she be counted? ... interesting...