Why does the supernatural make sense?
Zombies don't metabolize because they're dead. This means they're not digesting food. Which means that shouldn't eat at all.
Vampires come out at night because sunlight hurts them, but apparently moonlight – which is just sunlight bouncing off the face of the moon – doesn't give them any trouble.
Ghost are the spirits of the dead which live on beyond the extinction of their mortal bodies – but then why are some ghosts depicted as being clothed? Did their pants have a spirit that lives on beyond the extinction of their owner?
And, yet, it makes somehow makes sense that zombies crave brains, vampires work the night shift, and ghosts aren't all nudist.
Here comes the science.
The Frontal Cortex neuroscience blog has an interesting article on "double scope integration," or the human ability to spontaneously generate meaningful intellectual frameworks between two distinct, even contradictory, realities. Lehrer gives an example from the children's classic Harold and the Purple Crayon:
All of which leads me to Harold and the Purple Crayon, one of my favorite childhood books. (The book is written for three-year olds.) The conceit of the book is that Harold has a magic crayon: whenever he uses this purple Crayola to draw, the drawing becomes real, although it's still identifiable as a childish sketch. For instance, when Harold wants to go for a walk, he simply draws a path with his crayon - this fictive path then transforms into a real walkway, which Harold can stroll along. When Harold's hand wavers and he draws a mass of squiggly lines the end result is a stormy sea.
Harold is a perfect example of what's known as "double-scope integration". This is a fancy term for something we all do everyday, and have been doing since preschool. In essence, double-scope integration (aka "conceptual blending") is the ability to combine two completely distinct concepts or realities in the same blink of thought. For instance, even young children are able to seamlessly blend together the world of actual space-time (in which purple crayons don't create walkways or moons or oceans) and the world of Harold, in which such things are possible. The text only works because such cognitive mergers are possible: after Harold draws a new object, the rules of the real world still apply. So when he draws a mountain (and then climbs the mountain), he still has to make sure he doesn't slip and fall down. When he does slip - gravity exists even in this crayon universe - Harold then has to draw a balloon to save himself.
Lehrer then goes on to quote cognitive psychologist Mark Turner (who was among the first to seriously study the phenomenon):
Double-scope integration integrates two mental assemblies, two notions, two thoughts that conflict in their basic conceptual organizations, because they are based on conflicting frames or conflicting identities. The result of this integration is a new conceptual array, a "blend," that has a new organizing structure and emergent meaning of its own. In "double-scope" integration, there are two input menial spaces that we typically keep quite separate, but there is also the invention of a blend that draws crucially on both of them.
To keep the discussion in the wheelhouse of this blog, successful double scope integration is the key to why supernatural horror. Despite what would seem like a fatally flawed set of contradictory ground rules, supernatural horror can be easily understood and enjoyed because readers and viewers create ad hoc, temporary, and evolving bridge between the rules of the real world and the often limited rules of the imagined, supernatural world.
Take, for example, Slimer, the gluttonous green ghost that became the mascot of the Ghostbuster franchise. Though Ghostbusters is dubiously a horror flick, Slimer is indubitably a ghost, so he'll serve for our purposes here. Slimer is an excellent example of how double scope integration not only blends the fantastic and the real, but how this blend can evolve to accommodate new info.
In Slimer's first appearance in the first film, he's floating beside a room service cart, chowing down on the food arranged on the cart. He picks up plates of food and shovels them towards his mouth. Most of the food misses and tumbles down the front of his pear shaped, floating body. At this point, Slimer has the physics of a solid body. When food hits him, it bounces off and moves downward. He also follows a sort of common sense physics that suggests that the broader, fatter part of his body should hang lower and he should maintain an upright posture even though gravity seems to have no effect on him. Furthermore, when Ray attempts to capture Slimer, the beastie takes off and reveals that ghosts – though propelled by no visible source and using no visible means of locomotion – are subject to acceleration and deceleration. Not just that, but ghosts get winded. In the following scene, Slimer encounters Venkman and the ghost appears to be panting.
That a ghost can be out of shape, must adhere to the x-y-z of 3D space, and will exhibit inertia and momentum is somewhat silly given the creature's otherwise total disregard for biology and physics, unnecessary. But it makes the double scope integration easy. We're given enough of a "hook" to blend Slimer's behavior into the rules of the real world.
Once that blending starts, viewers and creators can even exploit a certain level of flexibility – even when the new info contradicts what we've already established as the ground rules of the supernatural event. In Slimer's case, Ray's attempt to blast him sends him rocketing down the long hotel corridor. Slimer then vanishes through the wall. Slimer is, somehow, intangible. This despite the fact that Slimer acted like a solid body before. After establishing his intangibility, the next time we seem Slimer consume food, it will pass right through him: In the ballroom scene, the ghost drinks a bottle of wine and it dribbles straight through his body and on to the table he is hovering above.
But, weirdly, we'll see Slimer eating once more in the film – briefly during the "ghost explosion" scene he appears from inside a street-meat hot dog cart – and he's solid again.
So what are the "rules" for Slimer? Is he solid? Is he intangible? Impossibly both?
Truthfully, it doesn't matter. The viewer is given enough info to double scope integrate and, once that's possible, there's little to be gained by adding complex ground rules in order to somehow reconcile the mutually exclusive concepts you're working with.
However, our capacity to double scope integrate does have limits. One of the classic traps of supernatural horror is to fall into narrative structure in which weird crap just keeps happening, but there appears to be no particular rhyme or reason to it. A great example of this is the subpar sequel to The Ring. Like much J-horror, The Ring encourages double scope integration by basically laying out it ground rules from the start. The Ring II is such a profound failure because it throws out those ground rules without ever giving us any replacement concepts to integrate with our knowledge of the world. The result is a series of scenes which, even when creepy, make no sense and do feel like a narrative.
So why does Slimer function when post-tape trapped Samara doesn't? I don't think there's a strict answer to that. That's why, even when science can give us insight into the process, it remains a question of artistry.