If you've just seen Up, as I have, you might think you could put anything in a movie or a story. Dogs flying airplanes? A house carried through the sky on kids' balloons? Greek legends tell us of Zeus turning himself into a stream of gold, a hunter into a deer, maidens transformed into trees, even into pure sound (Echo). Kakfa's hero in Metamorphosis famously turns into a scarab. Is there no limit to such transformations?

In an ingenious paper, Michael Kelly and Frank Keil, psychologists, studied the transformations in Ovid's Metamorphoses and the Grimms' fairy tales. In these stories, nymphs turn into trees and straw turns into gold and scullery maids turn into princesses. There are, in short, all kinds of totally unreal, improbable transformations.

What Kelly and Keil found was that these transformations could not just be any old transformation. They stayed within certain limits. You could, for example, write a story in which milk turns red; you couldn't write a story in which an idea turns red.

Philosophers speak of "predicability." That is, certain predicates can go with certain subjects and other predicates cannot. Milk, being a concrete thing, can be red. An idea cannot (as in, "Colorless green ideas sleep furiously"). One can go on, Kelly and Keil show, to set up a "predicability tree." Such a diagram arranges predicates to sort out which can go with which. A predicate such as "is nearby" is above a term if it can be predicated of that term, as "is nearby" can be predicated of a man or a rose or a refrigerator or milk or a kiss. It would be above those. But "is nearby" cannot be predicated of love or fear or an idea or "literature." It would be below those.

Kelly and Keil sorted out the transformations in Metamorphoses and the Grimms' tales and found that conscious beings could be transformed into other conscious beings, animals, plants, inanimate objects, liquids, or even events, but in all the stories no conscious being was transformed into an abstraction.

In other words, Terminator 2 could turn into a pool of liquid metal but not into "Thou shalt not commit adultery." That is, "is a liquid" could be predicated of a Terminator, but not "is one of the Ten Commandments."

Possibly these categories and the limits they propose are built into our brains. Children can distinguish animate from inanimate things at a very early age. These "predicability trees" may correspond to the well-established fact in neurology that there is a certain area in the brain (the inferior temporal cortex, part of the "what" system for visual perception) that is essential to naming and processing information about objects in specific categories, such as tools, animals, or edible materials. We know this because of the lesions of some patients. Some patients or some experimental subjects in whom particular areas of the inferior temporal cortex have been taken out of commission, lose certain quite specific abilities, to name a tool or an animal or an edible material or a verb. (These categories may relate to basic human activities like grasping or eating.)

Could it be that a "predicability tree" is wired in us? That we are wired not to accept some kinds of improbabilities in fictions?

Items I've referred to:

Kelly, Michael and Frank Keil. "The More Things Change... ; Metamorphoses and Conceptual Structure." Cognitive Science 9 (1985): 403-16.

Also, I admit to adapting this post from my new book, Literature and the Brain (PsyArt Foundation, 2009). Available at www.literatureandthebrain.com.

Recent Posts in This Is Your Brain on Culture

How Novels Make Me a Murderer

I want the killer, thief, terrorist, to succeed--why?

Who Says Film Is Dead?

Are movies dead? Not if you can help it.

All about Brain Scans

Find out all about brain scans—or almost all.

Moneyball: The Behaviorist Movie

Moneyball is really a movie about psychological paradigms.

Stories and the Mirror Inside You

You don't think your way into another mind. You embody.

Literary Darwinism, the Deer Mouse, and the Three-Spine Stickleback

Real evolutionary study refutes the claims of literary Darwinists.