Those who offer a straight answer about just "What is a Novel?"--beyond its being a "book-length" work of prose fiction--are (as previously pointed out here) apt to point to three qualities distinguishing the modern novel in the narrow sense from other kinds of long prose fiction, not totally unprecedented, but previously relatively rare, and much less likely to appear together in combination:
1. Its centering on the life of everyday, middle class persons, who had in pre-modern times received much less attention of this kind from storytellers, much more inclined to offer chivalric epics, royal tragedies and comedies of low-life.
2. What we might term a pseudo-documentary quality to the proceedings, which are supposed to appear realistically detailed in a straightforward fashion rather than conspicuously embellished and ornamented in the fashion of a romance or a picaresque.
3. An individualistic and even intimate approach to the tale, peering into an individual's private life, and even their innermost thoughts--which may not be limited in the manner of an occasional Shakespearian aside to the audience, but part of the "normal" way of telling the tale.
You can find all these in the brief preface to Daniel Defoe's Robinson Crusoe, the first two in particular specifically referenced. As he remarks, it is "the story of . . . [a] Private Man's Adventures in the World," while "The Story is told with Modesty, with Seriousness . . . The Editor believes the thing to be a just History of Fact; neither is there any Appearance of Fiction in it." The third is implicit in these, but becomes apparent soon enough when we turn the first page and hear the protagonist relate his upbringing, his aspiration to go to sea, his dialogues with his parents about this idea--in the course of which dialogue Robinson's dad also gives him a long lecture about the great virtues of "the middle state," into which he was born, for which he seemed destined, and which his father also thought most likely to make him happy.
Of course, the tale takes a more exotic turn than Robinson's settling down to the cozy bourgeois existence his father intended for him, but these fundamentals of the story define what follows nonetheless.
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What is a Novel?