If, however, you're more like me, and believe that human personality is nothing more (or less) than a congregation of chemical reactions; and that people have the capacity to be not only unpredictable, but fundamentally unknowable; and that nature still has plenty to prove to us -- then you're likely to compare Trent's concept to cow excrement and find it wanting.
It may be that the two ways of thinking cannot communicate with each other. That, to my mind, proves the second way of thinking to be closer to truth than the first (but, of course, I'm biased).
Or it may be that Trent simply forgot to say that he was not talking about all fiction, but only fiction that aspires to be "psychological realism", a popular mode of fantasy in 20th century literature. If that is the case, then he's entirely correct. One of the requirements of the genre of "psychological realism" is that characters have clear motivations that create a cause and effect structure whereby a character wants X, and so Y action follows. Such writing is a genre of fantasy because it bears little relation to reality, despite its name.
Trent says, "If you're going to write about living creatures we know and understand, you have to follow their rules. If they don't follow their own rules, they fall apart."
But people do fall apart. People do things that they don't, themselves, understand, and that 20 years of therapy may only been able to give them superficial stories for. People are unpredictable, bizarre, wanton, extreme, contradictory, and utterly messy. (Shoot a railroad spike through their frontal lobe and their personality will change entirely.)
Sometimes we think we understand things, but that's only because we tell ourselves stories. How often have you done something that seems odd, and said, "I did that because ________." But you know the blank is really blank, despite all the stories you fill it with. The stories are helpful, and they may illuminate a portion of the truth, but a core of mystery remains. It is exactly this mystery that makes much of the best fiction interesting, because the mystery prevents readers from reducing the characters to simple, unambiguous interpretations.
You don't have to be as devoted to chaos as I am to see the flaws of any general statement about the nature and purpose of literature. Everyone who says there are rules of fiction sets themselves up to be proved wrong. A handful of writers are usful for this task -- when faced with a prescriber, haul out Laurence Sterne, Franz Kafka, Virginia Woolf, Gertrude Stein, Jorge Luis Borges, or Italo Calvino (among others). One or all is likely to be just the example you need to disprove any rule.
Trent says, "You cannot have a pedophile on one page and magically transform him into something else the next. You can transition or alter the character so long as it falls within the boundaries of his rules for living." But a story about a pedophile who suddenly and inexplicably stops being a pedophile would be fascinating, particularly if the writer had the integrity to never offer a reason for this change of behavior. Such a story could be marvelously ambiguous, mysteriously real. Characters in the story might try out various explanations, might offer reasons and justifications and hypotheses and guesses, but none would be adequate.
Nathaniel Hawthorne's "Wakefield" is just such a story. It is brilliant because it avoids reducing its character to clear psychological motives. Of the title character, Hawthorne says, "Had his acquaintances been asked who was the man in London the surest to perform nothing today which should be remembered on the morrow, they would have thought of Wakefield." Yet Wakefield's wife thinks there is something a bit odd about him -- "what she called a little strangeness sometimes in the good man. This latter quality is indefinable, and perhaps nonexistent." (The wife may think she knows her husband, but the narrator has doubts.) Wakefield, the most ordinary of men, walks out of his house one day and gets an apartment one street away from his previous home, telling his wife he's going off on a short journey and isn't sure when he'll return. Why does he do this? He doesn't know. "Such are his loose and rambling modes of thought that he has taken this very singular step with the consciousness of a purpose, indeed, but without being able to define it sufficiently for his own contemplation." He gets a new wig and different clothes so as to disguise himself should his wife see him passing by.
The narrator in "Wakefield", though, holds out hope for an explanation of the protagonist's behavior: "Would that I had a folio to write, instead of an article of a dozen pages! Then might I exemplify how an influence beyond our control lays its strong hand on every deed which we do." But the story itself is not able to provide such explanations. After twenty years, Wakefield one day ends up standing on the doorstep of his old home, rain begins to fall, and he goes inside. The final paragraph reads:
This happy event -- supposing it to be such -- could only have occurred at an unpremeditated moment. We will not follow our friend across the threshold. He has left us much food for thought, a portion of which shall lend its wisdom to a moral, and be shaped into a figure. Amid the seeming confusion of our mysterious world, individuals are so nicely adjusted to a system, and systems to one another, and to a whole, that by stepping aside for a moment a man exposes himself to a fearful risk of losing his place forever. Like Wakefield, he may become, as it were, the outcast of the universe.Perhaps I am misunderstanding the term "consistency", and Wakefield is consistent because he doesn't become a pedophile. Or he is consistent because he does what we are told in the first paragraph he will do (the entire plot is laid out in that first paragraph; everything else is elaboration, told in the voice of someone imagining details to a tale half-remembered from a newspaper). If Wakefield is a consistent character, though, I can't imagine what an inconsistent one would look like.
"Consistency" is similar to "the unities" that Renaissance and Neoclassical critics proclaimed necessary to all drama. They claimed to get their authority from Aristotle, and though he was more descriptive than entirely prescriptive this didn't stop the totalitarian impulses of the critics, who enforced the view that all drama must have unity of action, time, and place. "Consistency of character" looks to be just another name for unity of action, which came from Aristotle's definition of tragedy as "an imitation of an action that is complete, whole, and of a certain magnitude", a definition most critics have argued calls for a beginning/middle/end structure and clear cause-and-effect relationships. (For a great study of all the various -- and doomed -- definitions of tragedy, see Terry Eagleton's Sweet Violence: The Idea of the Tragic.)
Plenty of writers have either intentionally or unintentionally written great work by utilizing one or all of the unities. Plenty of writers have either intentionally or unintentionally written great work by ignoring one or all of the unities. This has happened for centuries.
Some of the most interesting playwrights of the past thirty years or so have deliberately written plays with characters that could not be described as "consistent" under any definition of the word. One of my favorites is Mac Wellman, winner of the 2003 Obie Award for Lifetime Achievement, who has written plays that intentionally frustrate all desires for predictability and consistency. But if you've ever seen any of his work performed by a competent company, you know that it can be utterly compelling -- far more so than most of the consistent characters in what Wellman calls "geezer theatre".
In a 1997 interview collected in The Playwright's Voice, Wellman says:
Motivation is always boring to me. You need some of it to create narratives, but you need far less than many American dramatists think. The problem with motivation and foreshadowing is that if you do it in a straight-ahead way, the drama is always happening elsewhere. And it reduces all characters to something that may be plausible intellectually, but it's not very satisfying. All people and actions become understandable. That's why I find the work of someone like Arthur Miller ridiculous and shallow. Success and failure turn into these interchangeable icons and nothing is illuminated. A noble man, and certainly somebody I respect, but there are literally dozens of writers of that type whom intelligent people seem to find very moving.Later in the interview he quotes Simon Callow as saying, "For me it's not the great stories, or even the great language or the great drama, it's the great characters who can will exactly contradictory things at the same moment. That to me is what theatre is about." Then Wellman says, "A lot of my friends who are playwrights are interested in the strangeness and beauty of the world, which Ovid talks about: you can be a beautiful young girl one minute and then be turned into a tree stump."
Suzan-Lori Parks is another writer of brilliantly inconsistent characters, although she has also proved herself capable of writing perfectly consistent characters as well (she won a Pulitzer for a play about a couple such characters, although they're interesting enough to be not entirely consistent). Maria Irene Fornes, Len Jenkin, Richard Foreman, Jeffrey M. Jones, Eric Overmyer, David Greenspan, Christopher Durang, and many other playwrights have had great fun destroying, to some extent or another, clear motivation for their characters' actions, reveling in the wondrous chaos such destruction has created.
Trent Walters says,
Truth be told, you probably have an interest in something that makes no sense to the majority of the public. Maybe you like feet or shoes or large noses. These quirks are what make you you. It's natural because it exists in nature. Nature has created some beautiful, some strange, some hideous traits in its long life. That's the nature of nature. Any trait in nature should be free game for fiction.What is something that doesn't exist in nature? And who is to say that something doesn't exist in nature -- who is omniscient enough to know all of nature? Just because something doesn't make sense to you doesn't mean it's unnatural. What, are we supposed to limit our fiction to things explored by sociological, psychological, and anthropological studies? If something can't be verified by somebody in the Department of Human Studies at the University of All Things Proveable, it shouldn't be written about?
Write what you want to write, how you want to write it. If you like rules, then make some up and follow them. If you don't like rules, then fart in their general direction. But don't try to make everybody else play your little game, and don't pretend "the rules" are anything other than what we know them to be: guidelines that have been useful to some people in the past and may be useful to some people in the future. Following rules doesn't guarantee success any more than breaking them does.
Update 1/22: Trent has posted a great response. My goal was to be annoying enough for him to clarify his terms, and he has. Very nice.