Solipsism is the philosophical theory that the only thing that exists is the person who holds the theory. Everything else is generated by the mind of this person, and life can be thought of as a kind of lucid dream or hallucination. To ease comprehension, this entry is written as if the universe existed — things get confusing otherwise.
Arguments for Solipsism
The existence of the solipsist is normally argued on the basis of Descartes' famous statement Cogito Ergo Sum ('I think, therefore I am'). Beyond this, it is argued, nothing is certain. But we can say quite definitely that either the universe exists, or it doesn't exist — indeed, this is a logical tautology. To decide which we can use Occam's Razor.
Occam's Razor is an example of a metatheory — a theory which describes how to choose between different theories. There is some evidence that using Occam's Razor tends to lead to more accurate and useful theories, though it isn't something that can be absolutely proved one way or the other. The principle is that given two theories, both of which are equally effective at explaining the given evidence, we should 'slice out' the more complicated one.
If the universe exists, then it must have some non-zero complexity. So the solipsist viewpoint is the simpler of the two. Therefore, it is argued, we can invoke Occam's Razor to slice out the universe, and decide that the solipsist viewpoint is the correct one.
Problems with Solipsism
Solipsism is widely seen as a rather depressing view of the world, and it should come as no surprise that the argument for it is hotly contested. There are essentially four different ways that people argue against solipsism, depending on their viewpoint.
The first and simplest way is to claim that Occam's Razor is not a good method of choosing between models. Indeed, the suggestion that Occam's Razor supports solipsism is a commonly used argument against its validity.
The second objection is to the claim that solipsism is simpler than the universe. If solipsism is true, then all that we think is the universe must be a dream in our mind. However, for this to happen our mind must be large enough to simulate an entire universe. Something able to simulate the universe must be more complicated than the universe itself, surely?
Well, say the solipsists, it depends. If the simulation was inaccurate, perhaps not bothering with any of the detail beyond the solar system, then it could be simpler. This is, amazingly, a testable theory — if these people are correct then eventually, given clever enough experiments, we should be able to find glitches in the structure of the universe. There are even some who argue that quantum physics is the result of these glitches, in the same way that aliasing is the result of computer pictures being made up of pixels.
The third objection is that the theories are not equally good at explaining the evidence. If the universe is the product of one's imagination, people say, then why isn't it more to their liking? Surely they'd want to create a universe that smelt better, where beautiful men (or women) are throwing themselves at their feet?
The final objection is more pragmatic, and ignores the question of what ultimate truth is altogether. Instead, its proponents claim that it is more fun to live our lives assuming that the universe is real, and more of a challenge, regardless of whether it happens to be true or not. Given this, why encumber your mind with solipsism?
The weak version of solipsism is that the universe and everyone else exists, but the weak solipsist alone exists as an independent conscious being, and everything else in the world is an object, with no ability for independent thought. The argument for this position is very similar, but it renders the second and third objections to normal solipsism redundant.
However, this argument does have other problems — the main one being that it violates the principle that our position in time and space is not special or unique. Weak solipsism violates this principle in a huge way, because it claims that the weak solipsist is unique in being conscious.
Some people go beyond solipsism and claim that not only does the universe not exist, but nothing exists at all, including the person arguing this position. A more detailed explanation of this idea can be found in the Illuminatus! trilogy by Robert Anton Wilson. Essentially it is built upon similar arguments to solipsism, except that Descarte's argument for the existence of self is declared invalid.
Reverse solipsism is the claim that the reverse solipsist does not exist, but everything else does. The person arguing this position normally claims that whoever is listening doesn't think, so they can't prove that they exist. 'Think back', they say, 'when was the last time you actually thought?' Furthermore, it is claimed, no thinking person would be listening to such a total load of hogwash (this is the normal cue for the listeners to get up and get a beer). Meanwhile, everyone else in the world is getting on with their lives and being productive and thinking, so they do exist.
The Final Proof
From the Researcher of this entry:
See, you're still reading! If you were thinking, you wouldn't still be reading this. Evidently, I was right, you don't exist. Or rather, you do. From your perspective, you don't exist. From mine, you do. From your perspective, I exist. From mine, I do not.
There's no arguing with that, is there?