At the opening of Book 1, Chapter 7, Fielding—under the guise of reciting an ancient bit of philosophy—expounds, through simile, upon the nature of love and lust:
It is the Observation of some antient Sage, whose Name I have forgot, that Passions operate differently on the human Mind, as Diseases on the Body, in proportion to the Strength or Weakness, Soundness or Rottenness of one and the other.
In this simile, Fielding compares the effect that “passions” have on the human mind with the effect that diseases have on the human body—meaning that a sound mind will be able to resist the effects of passions like a strong body resists the effects of disease. As the rest of this chapter will concern Lady Booby, this opening remark is presumably a jab at the apparent "Rottenness" of Lady Booby's brain, evident by the speed at which she succumbed to her passion—which is to say, lust—for Joseph Andrews.
This is clearly a statement in support of Christian morality, in which lust is a sin, chastity is an ideal, and virtuous love is not necessarily marked by any sort of libidinal passion. It thereby fits neatly within Fielding's exploration of love, in all its forms, throughout his novel.
Throughout Joseph Andrews, Fielding interjects to expound upon the nature of novel-writing itself. In Book 2, Chapter 1, he explains the practice of dividing and subdividing his work into books and chapters through simile:
[…] I take this of dividing our Works into Books and Chapters to be […] considerable. Now for want of being truly acquainted with this Secret, common Readers imagine, that by this Art of dividing, we mean only to swell our Works to a much larger Bulk than they would otherwise be extended to. These several places therefore […], which are filled with our books and chapters, are understood as so much buckram, stays, and stay-tape in a taylor’s bill, serving only to make up the sum total, commonly found at the bottom of our first page and of his last.
In this simile, Fielding compares the structure of his book—with its formatting of individual chapters sorted into individual books—to a bill of sale from a tailor that has been artificially increased by little frivolous charges. This observation mocks the self-important, elaborate structure of more serious novels while emphasizing that Fielding's own incessant subdivision of his story is meant to be a satire of this practice. Joseph Andrews is full of these sorts of asides, where Fielding makes note of a convention, points out its arbitrariness or silliness, and then proceeds to himself employ the convention to the extreme in order to lampoon it further.
