Cognito ergo sum; I think, therefore I am.

- Rene Descartes

"The thing about ghosts," Maddie said conversationally, "is that they're psychological beings."

The ghost that was currently flying the both of them at dizzying speeds high over Lake Eerie looked down at her with a quizzical frown. "Well yeah, I guess." He glanced over his shoulder at the monster he'd whipped her away from just seconds before. "Except maybe that thing. It was nothing but green goo and bad attitude."

"Just like you," Maddie reminded him. "An amalgm of nothing but ectoplasmic energy and aggression."

The ghost boy huffed. "I save you from being fishbait and that's the thanks I get?"

"You left my son behind!" she snapped back, wriggling uselessly in his grip. He had her in the awkward and particularly vulnerable position of his arms tucked under her arms and gripping her shoulders, leaving her to dangle helplessly as he carried her away from Danny and what was left of their tiny fishing boat. She needed to go back. She had to save him. This wretched ghost simply didn't understand. If only she could reach her ectogun...

He chuckled and held her arms tighter, making the efforts useless. "Don't worry. Danny's fine."

Maddie scowled, but then she smiled slyly. She may not be able to destroy this ghost, but she had a plan.

"Ghosts are governed by their obsessive drive, their emotions, their beliefs. The only definitive part of them is their psyche. What they think not only defines their personality, but what they can do."

"So?

There was a splash and an almighty bellow. Maddie craned her neck and caught a glimpse of the beast behind them launching its bulk into the air. Its massive tail churned the normally quiet lake waters into whitecaps; she could just see the little yellow blob that was their boat, keel-up and bobbing wildly. Was Danny still in it?

"Crud," Phantom muttered, and flew faster.

Maddie had to shout over the wind resistance. "No one thinks about gravity, ghosts least of all! It's why they levitate. Because gravitational pull is not taken into account in their psychological makeup."

"This is great and all-" Phantom dove to the right. Maddie swung under him like a ragdoll; she could feel the fiery breath of the sea creature lick at her heels. "But can't the ectobiology lecture wait till we're not fleeing for our lives?!"

He dove and skimmed so close to the water her boots skimmed the waves. The leviathan was right behind them, fury in its glowing, beachball-sized scarlet eyes. Phantom whipped to the right and looped high in the air. The leviathan took a moment to swing its mass around to follow. The ghost kid had bought them some time; he'd also unwittingly brought her nearer to the boat.

Maddie went on relentlessly. "You must know the ectoplasm in your body is a semi-solid, not a gas. It has mass, and therefore weight. So what's holding us up?"

"I dunno, ghost energy?" She watched his expression closely; the ghost's dark eyebrows knitted together in a frown. He was starting to get confused; good.

"Really? That would be a lot of energy to keep us up so high. Even a jetpack couldn't have gone on for this long."

"Why are we even arguing over this? I'm holding you up, aren't I?" He slung her onto one arm and fired a couple of ectoblasts back at the beast, then grabbed hold of her shoulders again. "And you're freakin' heavy and hard to fight around, by the way, so you're welcome."

One of the blasts hit the creature's eye. It roared, falling back as it clawed at the injured socket, which wept greenish fluid. It dove back into the water and vanished. Maddie could feel the ghost above her breath a sigh of relief.

Until the leviathan appeared right beneath them, jaws spread wide.

Maddie had to shout over the roar of the water cast violently aside by the thing's massive head. "Humans have a much denser mass than ghosts. I weigh twice what you do, Phantom."

Its jaws jutted high on either side of them and blocked out the sky. The red cavern that was its throat tunneled down into darkness, gleaming with acid. Green-white teeth glistened like a city of ivory towers.

Phantom rocketed skyward, straining against his grip on her shoulders. They made it out just as the jaws snapped closed. She could feel the impact vibrating just below her feet.

"Aren't girls supposed to be shy about their weight?" Phantom managed another shot over Maddie's shoulder and struck the other eye, leaving the beast thrashing and blind.

Maddie glimpsed the boat. It was barely a hundred feet away.

"Don't change the subject. That weight doubles your burden...and doesn't answer my question. It's literally impossible for you to do what you're doing right now. "

They wobbled just a little bit and sank a few inches in the air. Maddie pounced on Phantom's uncertainty. "In reality, gravity is pulling on both of us. You might get a little bouyancy out of your ghost powers, but me? I'm dragging you down. You're going to fall. And you know it."

"I didn't say that! You just confused me! That doesn't even make sense, because...because..."

He must have made up his mind, because the effect was immediate.

"Hoooly-I can't-what did you do?!" Phantom shrieked as they plummeted from the sky like a stone.

Of course everything she'd made up was nonsense; ectoplasm was a transdimensional substance and didn't follow the rules of physics, whether or not the ghost was psychically inclined to levitate. But Phantom had believed her, if only for the moment...and ghosts are governed by their beliefs.

The water rushed up to meet them. Maddie grinned and reached for the Fenton Flotation Device. She was going to rescue her son, and no gullible idiot of a ghost was going to stop her.

A/N: It's been entirely too long since I've posted on FFn, so here's a quick little oneshot. This was written for Becca's Phanniemay challenge on Tumblr, originally titled "Cognito Ergo Doom".

I could see this as being a fun comedy-of-errors kind of short story, but considering all the other fics on my plate at the moment, the chances of me actually continuing it are 0.00001%. Guess I'll have to leave it up to your imaginations. ;)

Thanks for reading!

-Hj