Solipsism is a philosophical belief that only one's mind is certain to exist. You, reading this: Think, right now. No, just think. By thinking, you have confirmed Rene Descartes' immortal postulate—"I think, therefore I am"—and proved that you exist. Now, can you prove that anything else around you exists? Let's start with the digital screen upon which you are probably reading this treatise. Can you prove it exists? You might say, "I am seeing it, therefore it exists..." But what you are seeing is not necessarily what is actually there. Your eyes perceive light and convert that perception into nerve impulses, which are then compiled by your brain into some sort of visual picture. With all these translations going on, are you sure that what your brain perceives is actually there? It is very accurate to say that the computer screen is projecting something which your brain perceives to be text, but you cannot, with your eyes alone, prove that the computer screen is actually projecting text. And here's the scary thing: neither can anybody else. Even if you ask your friend to come over and confirm the existence of the thing, the problem is that his or her eyes and nerves are just as fallible, just as untrustworthy, as yours. In other words, while the two of you can largely agree on the mass hallucination you call "a computer screen," it still is a mass hallucination. Maybe the computer screen is projecting something entirely different. Maybe what we call "blue" is actually orange. Maybe the computer screen isn't there. Something must be there, to create the physical phenomena we call "computer screen," but there is nothing to guarantee that said thing is anything like what we call a computer screen... or, indeed, is anything like anything at all. Heck, maybe it isn't there! We still can't verify the thing's qualities; how can we possibly verify its existence?? It may actually exist—it probably exists; otherwise the fact that we are seeing the contents of a TV Tropes Useful Notes page printed on thin air says something rather unflattering about our Sanity Meter—but we cannot prove it. And let's take it a step farther: now that you've asked your friend to prove that the computer screen exists, can you prove that s/he exists? After all, your eyes and nerves are still just as fallible as they were when staring at that alleged computer screen... Ladies and gentlemen: solipsism.
Epistemology, Metaphysics and Methodology - or, "Huh??"Philosophy is a complicated beast, involving multiple branches, disciplines and areas of study. They include "Epistemology," the nature of knowledge, and "Metaphysics," contemplation of the fundamental nature of the world and those things in it. Epistemology asks, roughly, "What can we know, and how can we know it," while Metaphysics asks, "What, in the end, actually exists?—and, now that it exists, what is it like?" This is being brought up because solipsism has application to both branches. You've already seen how in the explanation above; now we'll elaborate a bit more.
Epistomological SolipsismSolipsism was first proposed by a Greek philosopher, Gorgias, some time during the 400s or 300s BC. He stated, very simply:
Metaphysical SolipsismThe most extreme version of solipsism, Metaphysical solipsism goes on to assert that, because we can't prove anything else exists, they therefore do not. Each of us might well be a Brain in a Jar for all we can verify about the outside world. We are all living in a mass hallucination—or rather, I am living in one, since I can't verify your existence and therefore you don't. I am the last of my kind. In addition to being a rather bleak prospect to contemplate, this philosophy lacks evangelical oomph: why, after all, would I try to convince you that you don't exist? In addition to being rather insulting to you, the simple fact is that you don't exist—you're a figment of the mass Imagine Spot I've dreamed up to distract myself from being a Brain in a Jar—and thus whether you believe in your own existence or not is just about the least relevant thing in the world. And even if I do succeed in converting you, you still don't exist, and my triumph cannot really be called meaningful. Even worse, there are profound effects on one's view of morality. It doesn't change to Black And White, or even Blue And Orange: it ceases to exist. Murder, for instance, is typically described as the unlawful killing of another person— What other person? Surely it is not immoral to kill an Imaginary Friend, who does not exist (and indeed, never existed) in the first place. The solipsist cannot cross the Moral Event Horizon because there is no one to condemn him or her for doing so. The end result is Video Game Cruelty Potential played deadly straight. In the end, metaphysical solipsism isn't a very practical philosophy: anyone who advocates for it either disavows his own belief or is Talking to Himself. However, the Brain in a Jar premise has proven very fertile through the years, and metaphysical solipsism has proven a useful case study for psychologists seeking to understand Lack of Empathy.
Usage in Fiction