AN: This story continues near the end of chapter 10 of Rationalising Death by Scientist's Thesis, which is a very good rationalist Death Note fanfiction. The first paragraph is taken directly from that story, up until the ellipsis.
What if she was in a plain old piece of fiction written by a plain old human? She didn't feel fictional, she felt very… very real. Deep, complex, human. ... Wait.
Before she could contemplate the ridiculous notion that writing a story gave that world some sort of "spark", an entirely different, much scarier idea came to her which ground her thought process to a short halt.
Her original argument against her own fictionality was that she didn't feel fictional. This evidence amounted to her wondering if she felt conscious, doing a deep mental enquiry on the question, determining that yes, she was conscious, and from this concluding that she couldn't be fictional.
But what if she was fictional? What would the query "Do I feel conscious?" return? The answer was that it would return yes. And while this would be lacking the deep mental enquiry that would take place in the real world, a fictional character wouldn't know that it was missing. A writer would instead replace the deep mental enquiry with the belief that such an enquiry had taken place, and the character would be unable to tell the difference.
Of course, this didn't prove that she was fictional. It only showed that her previous reason for dismissing her own fictionality was flawed. But that in itself was enough to skyrocket the probability she assigned to being a mere character.
After all, there were no known computer simulations of humans in this world, but there were a lot of stories. In all probability, this would also be true outside the box, where people were either simulating this world or writing about it. Plus, it was very likely that if computer simulations of humans became possible in a world similar to this one, it would be both heavily regulated and expensive, at least compared to writing. On top of that, stories were many times more likely to contain shinigami than a simulation, because stories were for entertainment, whereas simulations were much more likely to be used for research. And finally, there was the fact that simulations contained stories, but not vice versa, which meant that for every simulation out there, the number of stories increased by billions, drastically increasing the odds that any given "person" was fictional.
So while being in a simulation was possible, that possiblity was overshadowed by fiction by such a large margin that it was barely even worth considering anymore.
Then there was the other possibility which she hadn't liked from the beginning. The possibility that the world wasn't natural, and that magic was a fundamental part of the universe. But now she realized that that was even less likely than the simulation hypothesis, because it required many things she believed to be false, whereas the other two hypotheses were both possible without requiring magic at all.
Even Occam's Razor agreed with her. Either she lived in a world where there was both fiction and simulations/magic, or in a world where there was fiction. Which is simpler?
In the end, the conclusion was clear: she was a fictional character.
She realized the plot probably had something to do with the shinigami and Kira, but she couldn't bring herself to care anymore. An emotion she would describe as a heavy weight in her chest had settled on her when she had realized that none of this was real, that she didn't matter, and that as soon as the author got bored with writing about her she would cease to exist and no one would shed a tear because she didn't actually feel anything, only thought she felt things. It was with no amusement that she realized that the outside world would probably classify her as a philosophical zombie.
She spared a thought for the others in her world. Most of them had likely never existed. But the select few who played a role in this tale, like Kira and Rem, were probably in the same situation as her, even though they were unaware of it. Her chest constricted tighter.
Despair, she thought. That's the name of this feeling. But am I really even feeling it? I think I feel it, but...
What would happen to the plot now that she felt this way? It was likely that this was not regular fiction, but a short fanfiction, so it had probably been completely derailed, actually.
Oh no, she realized. I'm going to die in just a few moments, aren't I? Unless by some miracle I turn out not to be fictional. But it doesn't help that my entire thought process on this topic has been clear and concise, as though it was exposition for an audience. On second thought, it was concise, but probably not clear. Odds are the author isn't eloquent enough to actually make it clear, even if it seems clear to me.
And it was then that she realized through divine intervention that this was really it, that as soon as this run-on sentence was over, she would be dead, and wasn't that thought both ridiculously scary and solid proof that she was fictional, and (oh god I'm dying) there was absolutely nothing she could do about it, and in the end it didn't really matter if she died, after all she wasn't real, (oh god no I don't care that you think I don't matter I think I matter and I really really don't want to die) but it was all for naught, as this sentence wound down to a close, and everything became nothing.
AN: So, what probability do you assign to your own fictionality? Keep in mind that even if you think you're conscious and non-fictional right now, maybe the author is just feeding you that illusion so that you don't realize you're fictional. If you're still not convinced, just wait until the day you die and wake up in the Naruto universe. I hear that's been happening a lot lately.