If solipsism held validity...

First, for those that don't know...

sol - ip - sism [sol-ip-siz-uhm]: Noun; Philosophy. The theory that only the self exists, or can be proven to exist.

The usual explanation is that anything and everything we see, hear, feel, experience, is all in our heads. Reality is simply our mind's interpretation of electrical signals. All of our friends and family could just be hallucinations, the reality we see is only the 'reality' that our mind gives us. There's no telling that other people even exist, because we only know what we perceive, and even that is questionable sometimes.

It's a common theory, really. The first thing that comes to mind is The Matrix idea, where we're all trapped in a computer simulation. The thing with this, though, is it isn't just us. I'm not even talking about the other people, yet. The thing with a simulation is that it feeds the mind. The things you experience aren't created by your own mind, they're injected by the simulation, so it isn't your brain feeding your brain. There are constants in such a world.

Another movie that touches on solipsism is A Beautiful Mind. Entire facilities are exposed as figments of the protagonist's mental condition, not to mention the hundreds of people and conspiracies that don't even exist. The problem with this, though, is that the conclusions he reaches and the patterns he finds are all things that he's expecting to be there. There are no curveballs thrown at him. If you really think about it, nothing happens that he wasn't expecting, in one way or another. Although, it is an excellent illustration of the mind feeding itself. The conclusions he reaches change his entire reality, and this new 'reality' of his spurs on further thought. It's essentially the human thought process and pattern recognition in the most visual form possible.

But for solipsism to be valid, there can be no outside influences, simply because there isn't an outside. What made The Matrix outstanding was when someone left the world. Without that, the whole trilogy would fall apart. What made A Beautiful Mind work was when the real people, the outside influences, made him question his reality. Without these outsiders, there is nothing to make people doubt. Without doubt, not only is there no desire to find factual evidence of solipsism, but there isn't a need to.

Without the doubt, it doesn't matter if solipsism is valid or not. If it is, then the believer will not notice it. They will go on living, and because they believe each and every person to be themselves, they will be gathering new ideas, new experiences, things they never knew existed. Things will happen that seem impossible. Man on the moon, quantum mechanics, wormholes, inter-dimensional travel. If solipsism was to be true, then none of these would seem impossible, and with enough thought, a person would be able to deduce how they should have already known that. If the world was all in our minds, then, with enough thought, nothing unexpected would happen. But this doesn't necessarily prove it false.

The people that find that doubt will seek out answers. They may find some, they may not. But in either case, no one can convince them otherwise. To the doubters, everyone is imaginary, and therefore, only offer differing viewpoints, but no new information. The people will likely be telling the doubter that they're wrong, presenting doubt to the doubt. From this perspective, the doubter doesn't want to doubt, as their "subconscious," or other people, are telling them otherwise. Reality becomes a dream the dreamer cannot wake up from, but this does not necessarily prove it's existence.

If they do manage to "wake up" from reality, it would put their consciousness somewhere else. If it's a populated world, it could prove solipsism false, or the mind could just be populating the new-found void with its projections. If the latter is true, something would need to have changed to make the doubter accept the world, or the process would repeat endlessly. Something inside the mind would have altered, or erased, part of itself to allow the doubter to continue living. Even this, though, is a change brought on by "waking up," and while the doubter will not be conscious of it, they will have changed. If the doubter is changing without their knowledge, just as the world previously existed only as projections of their mind (without their knowledge), then there is no evidence that even the self is real. Solipsism eventually caves in on itself, to the point where nothing is real. If  nothing is real, if it is all illusion, then there is no meaning to anything. Solipsism collapses to nihilism.

Without any meaning, the only purpose life has is to end, and in death, all preferences will have been broken down. Nothing will matter. There will be no good or bad, nothing more desirable over less. Life will not prevail over death, and so death will be the endpoint of nihilism, and, through association, solipsism.

Simply put, if solipsism is the question, death is the answer.