Okay, so we have this thing, and we call it reality, right?
There is no reality, and none of you exist. I'm not saying this from an ivory tower -- I don't exist either. Now, I'll tell you why.
Way back in the day some dude named Renee Descartes spend enough time in bed to come up with the idea that nothing exists. They called this Cartesian Doubt, because apparently in all of history nobody else had been bored enough to come up with that idea (yeah, right...). Now, he writes this book on it, but here's the clincher: half way through, he says "but, I'm thinking, yeah? So that means that in some form, I exist!" And there was much rejoicing, and he pissed all over his own fallacy. Unfortunately, he was born before the period when computers and postmodern philosophy and game theory and so on were commonplace, so he thought that it was obvious that anything that thinks exists (NB: in fields like philosophy, claiming something to be obvious, while mayhaps useful in the short term, is a pretty good way to make a total ass of yourself later). But, of course, he was wrong.
Take, as a counterpoint, the philosophy known as Externism. Externism is the direct inverse of Solipism: in Externism, everything exists except for the self, and the self only appears to exist because its surroundings skirt around this nonexistent thing, just as a hole in a piece of paper will change shape when you wiggle the paper around. In such a way, a thing that does not exist can not only appear to exist, but have a certain level of complex thought and interaction with its environment. However, if existence is simply constituted by the connective level, wherein by interacting with someone who does exist you automatically exist, then Externism fails. However, another even more obvious example looms on the horizon.
There are things that do not exist that influence our lives daily. In fact, one might argue that the only things that influence most people's lives are the things that don't exist. Since one must model one's universe to survive (it's not very useful to try to find food if you can't remember what food looks like and how to differentiate it from predators, the ground, the moon, and your own feet), we as humans have developed quite advanced modeling capabilites that are mostly transparent to us. When we see a chair, we see a chair, not a bunch of lines and colours jittering around with every saccade, and we have enough information in our world-model about chairs to know that it probably has a place you can sit on even if you can't see that place initially. We don't necessarily differentiate our mental model of a chair from a real chair, or at least we don't unless it doesn't match up. If we sit on it and we fall to the floor, it no longer matches the model of a chair, and we have to probably switch models (I would suggest putting that into a model of a broken chair, or if this happens too often, an unsturdy non-weight-supporting chair). This extends even into our social life, since by having totally imperfect yet useful models of people, we can avoid some pretty awkward social situations and sometimes predict reactions in other useful ways ("That guy is a go-getter, so if you propose this idea to him, he'll probably be pretty honest in appraising it. If it's good enough, he might even invest!").
Of course, our mental models, even of individuals, even of people we are close to (or even of ourselves) are typically quite inaccurate, and at best imperfect in ways that are of minor practical importance. However, the practical importance aspect doesn't negate the fact that the people we know and the people we think we know aren't the same people. We ourselves aren't even who we think we are. There are so many automatic layers of translation and generalization and pattern matching between our senses and our perceptions that we might as well just say the world we live in is entirely fabricated and be done with it.
But, so many people perceive the world in more or less the same way, you might say. How can that be, if our world is nearly entirely fiction?
I could take the easy way out here and say that we don't live in comparable world, but we only think we do. This is arguable, and I think it's true much of the time, however it doesn't do a good job of explaining why such a significant amount of our thoughts and perceptions are even communicable.
So, instead, I make an obscure reference to the phenomenon known as "folie a deux". This is an old term, as you can tell by the fact that it's in french and not some greek-latin-gobbledygook that would make your high school etymology teacher want to gag herself on a dirty dishrag printed with apostrophe errors. What it means, essentially, is a shared delusion or hallucination. It's documented, it happens a lot, etc. Apparently, we all give off a lot of subliminal cues about stuff that we don't realize we're giving off, and we pick a lot up and pass them on without ever realizing they're there. Some of this is easily explained via elementary cognitive science (word choice can prime interpretations which can trigger one belief over another, bla bla bla, ewige blumenkraft fnord), but the bottom line is that people somehow can share pretty detailed mental models of hallucinatory objects that don't even exist.
So, if much of our everyday perceptions are an (albeit quite useful) self-generated hallucination, and hallucinatory objects can be shared and perceived by a group simply by the exchange of signals in terms of word choice and body language, how do we know that a large portion of the objects and devices we use every day even exist? It's quite possible, for instance, that there are no real iPods in the world (to pick an arbitrary object of great popularity), and that Steve Jobs is unknowingly selling empty boxes that people buy and then hallucinate that they are listening to music on imaginary earbuds. Or, perhaps the building you work in every day doesn't actually exist, and the sensory info that makes you think you are there is actually a delusion. Or perhaps, maybe, our bodies and the physical world don't exist at all. Maybe in mind even, we are just hypotheticals -- characters in some novel that may never even be written, thinking in our first person narratives all day and never daring to consider that what we choose to do, what we eat, what we watch on television, how we do in school and what we do in public, don't matter in the least, because the story ends tomorrow and we're all minor characters who don't deserve a proper conclusion.
I hope this has been some food for thought. I'm not telling you to go out and do something crazy to spice up some hypothetical author-deity's ranking on the new york time's best seller list; I'm just trying to get across to you that so many things you have never questioned you could be entirely wrong about, as could anyone. No one knows the truth, and no one even knows if there is a truth. To put it bluntly,
no one knows.