After establishing his credentials as an historian, the narrator proceeds to list the qualifications necessary to write such histories as his in the introductory chapter to Book IX. Beginning with Tom Jones, Fielding "begins to educate the public about the craft of writing a novel rather than simply attacking one that he deems bad" (Slagle 191).
The qualities Fielding cites as "necessary to this order of historians" are (1) Genius, (2) Learning, (3) Conversation, and (4) a Good Heart (Fielding 424-26). Genius consists of invention ( "discovery or finding out"), and judgment (Fielding 424). This qualification expands on the idea that one should not judge someone without having true knowledge of all the circumstances surrounding the action, and the underlying motives. This admonition to the reader is displayed in Bliful; on the surface, his actions seem selfless and honorable the majority of the time. But the narrator lets the reader into the secret of Bliful's motives and so one sees that he is not selfless and honorable, but malicious and jealous. And so we are warned not to judge people by their appearances only.
Learning is, naturally, the sort one obtains through books and formal education. Conversation is learning of human nature, in all its varieties.
So necessary is this to the understanding the characters of men, that none are more ignorant of them than those learned pedants whose lives have been entirely consumed in colleges and among books; for however exquisitely human nature may have been described by writers, the true practical system can be learnt only in the world. (Fielding 425)
This observation goes to further show that appearances can not always be counted on as a guide to one's character. It is only through extensive conversation that people will reveal their true identities. But one must have the necessary sagacity to be able to discern the undercurrents existing in people's conversations.
The fourth requisite, a Good Heart, is, simply put, empathy towards others. Fielding clearly proves, with his narrative commentary, that one must be able to put oneself in the other's place before attempting to pass judgment on their actions. Also by this qualification, Fielding "invites us to enlarge the boundaries of our sympathy, so as to include the ridiculous" (Wright 42). Fielding, in the foregoing chapters, has steered readers into sympathizing with people suffering misfortune, and now begins to teach us how to empathize with characters that are plainly comic (like Partridge). We do not sympathize with Partridge because of the misfortunes in his life, rather we are drawn to laugh at his absurdities and can not help having a secret liking for him. Tom is well aware of Partridge's comic status, as indicated by taking him to see Hamlet so as to be amused by his reactions to it (Fielding 752).
In addition to setting down qualifications for writers of histories, Fielding elaborates on some of the stylistic techniques that may be used and those that should not be used. These are numerous, but only three will be discussed here: the "marvelous," the "supernatural," and "plagiarism." In the introduction to Book VIII, Fielding sets forth restrictions to writers as to the use of the 'marvellous' and the 'supernatural' (Fielding 346). First, he asserts writers must stay "within the bounds of possibility" and "probability" (Fielding 346). Also, Fielding cautions writers to use ghosts, which is the "only supernatural agents … allowed to  moderns" very sparingly (Fielding 347). It seems clear here that Fielding is attempting to justify his coincidences that occur in the novel that may seem extraordinary to readers. He therefore wants us to accept that these coincidences, as surprising as they may be, are in the realm of possibility and probability.
This preface sets the stage for the appearances of Partridge and the Man of the Hill, which do have a touch of the marvelous about them. Partridge is the man supposed to be Tom's father, which he fervently denies to Tom, so Tom's meeting with him does seem rather coincidental and forced by the narrator. Partridge also brings the supernatural elements into the narrative, being extremely superstitious and terrified of ghosts, witches, etc. While Fielding never actually brings any ghosts into the narrative (unless one counts the ghost in Hamlet), he does cite them indirectly through Partridge's fear of them. But this is most likely intended "to burlesque the superstitious faith" of Fielding's countrymen, as the narrator attributes to being the reason for Homer's overuse of supernatural agents (Fielding 347).
Partridge's extreme fears lead into the introduction of the Man of the Hill and his servant. Partridge is convinced the servant is a witch, and the narrator admits that if the servant "had lived in the reign of James the First, her appearance alone would have hanged her, almost without any evidence" (Fielding 385). Likewise, the Man of the Hill terrifies him:
This person was of the tallest size, with a long beard as white as snow. His body was clothed with the skin of an ass, made something into the form of a coat. He wore likewise boots on his legs, and a cap on his head, both composed of the skin of some other animals. (Fielding 388)
The Man of the Hill chapter is integral to the plot of Tom Jones insofar that the narrator gives the reader a description of a man who has become so entirely disillusioned with human nature that he "cuts off all contact with" other people, and so " deprive [s] himself of his own humanity "(Mandel 29). This loss of humanity is exhibited through the manner of his dress (animal skins). Manuel Schonhorn interprets the Man of the Hill episode as showing contrast between him and Tom. The "Man of the Hill's vision of the depravity of man is countered effectively by Jones's faith in the basic decency of human nature" (Schonhorn 210). This chapter underlines the narrator's assertion that human nature can only be discerned through conversation with all kinds of people. The Man of the Hill erroneously judges all humankind by the characteristics he has observed in brief conversations with just a few people.
Another stylistic technique that the narrator lectures writers and readers of historians about is borrowing words and phrases from other writers. He claims that all moderns can cite the works of the ancients with no restrictions whatsoever: "the ancients may be considered as a rich common, where every person who hath the smallest tenement in Parnassus hath a free right to fatten his muse" (Fielding 540 ). But borrowing from other modern writers would be considered plagiarism, so the narrator promises that if he does use the words of another modern, he will "put their mark on it, that it may be at all times ready to be restored to the right owner "(Fielding 541). As he does for them, so he expects them also to do likewise, viz. To give credit to him if they use his words.
Fielding explains his reason for the discussion of plagiarism as being so that his "conduct" will not be "misunderstood by ignorance" or "misrepresented by malice" (Fielding 538). He maintains he does not want his readers to misunderstand him or misrepresent his words, but nonetheless Fielding deliberately attempts to test the readers' sagacity by altering quotations slightly, eg a quotation of Pope's on page 184 and a quotation from Milton's Paradise Lost on page 288 . Fielding also sometimes attributes quotations to an author that in reality belong to Fielding himself, such as when he says Virgil compared "the mob" to an "ass" (Fielding 558). These little tricks of language that Fielding employs serve as a form of an 'in joke' for readers who are sagacious enough to catch on to his hidden meaning; thus, one is induced to identify with the narrator. Once Fielding has captivated his readers by his narrative style, he begins showing us how to be 'sagacious readers.'
Fielding, Henry. Tom Jones. Oxford: Oxford University Press, 1996.
Mandel, Jerome. "The Man of the Hill and Mrs. Fitzpatrick: Character and Narrative Technique in Tom Jones." Papers on Language and Literature: A Journal for Scholars and Critics of Language and Literature 5 (1969): 26-38.
Schonhorn, Manuel. "Fielding's Digressive-Parodic Artistry: Tom Jones and The Man of the Hill." } Texas Studies in Literature and Language: A Journal of the Humanities 10 (1968): 207-14.
Slagle, Judith Bailey and Robert Holtzclaw. "Narrative Voice and 'Chorus on the Stage' in Tom Jones." The Cinema of Tony Richardson. Eds. James M. Welsh and John C. Tibbetts. Albany: State University of New York, 1999. 189-205.
Wright, Andrew. Henry Fielding: Mask and Feast. Berkeley: University of California Press, 1965.