A Wonderful Long Chapter Concerning the Marvellous; Being Much the Longest of All Our Introductory Chapters
The narrator warns us that we're about to head into the part of Tom Jones where bizarre things happen. (So that's different from all the other parts—how, exactly?)
The narrator thinks it's his duty to talk a little bit about what readers can be expected to believe, and what they can't.
(1) No Supernatural Hocus Pocus
In classical poetry, authors often brought in gods to influence the plots of their stories.
Nowadays, writing that includes lots of gods does not exactly seem realistic.
The only supernatural actors who might come up in modern writing? Ghosts.
But ghosts definitelyhave no place in serious fiction. Fielding is a genre snob.
He won't even mention elves and fairies. (So—not a Tolkien fan? Ugh, this guy won't let us read anything fun.)
So, if we're ruling out gods, ghosts, and fairies, we have to concentrate on human beings as the best subjects for fiction.
(2) General Believability
Compared to historians, fiction writers have the worst problems staying within the bounds of what actually seems possible.
After all, if you're a historian, then you can find real-life proof that whatever crazy stories you tell are actually true.
If you're an inventor of fiction, you don't have any evidence to offer for your story lines.
So you have to stick with what your readers will find probable.
(3) Specific Believability
It's not enough to make sure that your plot lines are believable.
You also have to be certain that the actions of your characters appear (a) possible, and (b) possible for that specific character.
(So, if someone wrote a story based on Harry Potter with Slytherin's Severus Snape supporting Gryffindor at a Quidditch match, we would think, no way! That's impossible. It may not be physically impossible for Snape to wave a little Gryffindor flag and cheer for Gryffindor. But it would literally goes against everything we know about his character's feelings.)
You have to know human nature really well to make your characters three-dimensional in this way.
Within the limits of these three rules (no supernatural stuff, plot probability, and character consistency), the writer can be as inventive as he wants to be.
Just because your work should be probable doesn't mean it has to be boring.
You still have to find ways to surprise the reader.