Descartes asserted (as an axiom) that he could not doubt his own existence because to doubt implies a doubter. Everything else, the entire Universe itself, might exist only as a figment of my overheated dream-state imagination, a Matrix-like existence simulated for an audience of One, leaving him, like Pink Floyd's character in The Wall, wondering ``Is there anybody out there?''
One possible answer is no.
It is well known in philosophy that this belief is logically unassailable. How can you prove me wrong? First of all, you are a figment of my overactive imagination, a bit of beef gone bad, as Scrooge might say, so only I can prove me wrong13.12
This (solipsism) is the worst of all possible Bullshit philosophy. It is the kind of thing that makes ordinary people think of philosophers as Jackasses (according to the previous definition). It was the sort of thing that Johnson was once cheered for ``disproving'' (not really, of course, but who cares) in open debate by banging on a table (where he should probably thumped his opponent's head, although that would only have proven that he was a masochist, not that he was wrong).
There ought to be something wrong with it but it appears invulnerable even as it is absurd. What is really wrong with it?
For one thing, it is not terribly easy to reconcile true solipsism with our experiences of the Universe, at least when those experiences include Pain or Pleasure. It might answer one question (what is the nature of the universe) by an axiomatic fiat consistent with your one real empirical observation - that you exist there is no doubt if you try to doubt it, but it is only an axiom when you assert that only you exist. An axiom that is very difficult to reconcile with a number of provisionally and conditionally observed facts without more axioms, this time about the diseased state of your own mind.
Take me for example (since I'm certain that I'm here and you might be figments of my imagination).
If I am the only thing that exists, and the Universe is My Oyster served on a figmental half-shell, then why cannot I be surrounded by beautiful houris who do nothing but peel me half-naked grapes (or peel me grapes, half-naked, better yet) at a whim? Why do I have to plod along typing this instead of just wishing the lines onto the page? You see, Solipsism alone isn't a sufficient axiom. I need more. I need axioms to explain why I sometimes hurt, why my eyes are gradually failing as I age, why I age, why bad things happen to me. Sometimes very bad indeed.
I also need axioms to explain why my perceptions of what is nearby are so limited, but my perceptions of what is going on thousands of miles away through the glass teat of a television tube are crystal clear, complex, different, and correspond perfectly to what I see when I visit Paris, the Parthenon, India. Why and how I manage to be some sort of split personality.
If all of this is a figment of ``my'' imagination, then I've successfully managed to split myself into at least two incredibly separate beings - the artist that is constantly making up the story that I find myself embedded in, and the audience (the ``me'' that is typing this on what appears to be a laptop computer obviously created by my artistic half). The watcher within that watches the watcher watching, so to speak. Since I never perceive the artist directly, how do I know that it is ``me''?
Indeed, consider the artist further. I run computer simulations of physical models as some of the research that I do in physics. In these simulations, I ``create'' a virtual world of microscopic entities. Each is labelled with coordinates that specify the ```state'' of my little mini-world. There are rules whereby they operate. Computer games played by my children are very similar, at a higher order. They hold a virtual terrain superimposed on their internal coordinates, and have many ``sprite''-based components and characters. Those characters, objects, devices all have independent programmed personalities, probabilistic behaviors, an underlying ``physics'' of their interaction with each other and their surroundings, and a ``plot'' that unfolds as the game proceeds. I am not my computer models, the computer games are not my kids.
As they increase in complexity, to the point where a whole world is perfectly simulated with perfect consistency, the artist itself complexifies, its non-audience ``self'' splitting up among all the virtual selves it creates. If all of these (you who are reading this, and your dogs and cats too) are really part of the artist, and the artist is equated with the audience, then Solipsism is isomorphic to Pantheism. We are all God, split into all that is. Somehow a Western Solipsist (driven to explain why he cannot bring a loved one back to life no matter how hard he tries) ends up as an Eastern Hindu, accepting that Brahma split himself up to create the Universe (one fragment of which is him, all of which is still Brahma and eternal).
, but now you are wise and see the game I am playing with you. Axioms are neither true nor false, they just are. Premises, that is. Logically there are many ways to convert one into another, adding an axiom here, altering an idea there, ultimately dividing by the zero that is their informational content and proving whatever you like in conjunction with an undefined null set. Axiom sets can be inconsistent. Axiom sets can be consistent (or not), but they or the conclusions derived from them may not correspond to what we directly experience (and hence require special axioms to resolve the conflicts, which are then overcomplex and ugly).
So we see that Solipsism isn't logically provable. Sure, it isn't disprovable either (really) but nothing is provable or disprovable so that is irrelevant. What is (to me, and I hope to you) are two things. First of all, when somebody starts the Solipsism Game with you (where they assert that they are all that exists and dare you to disprove them) whack them upside the head - not too hard now - and cry out ``Bullshit''13.13. Second of all, not disprovable or not, Solipsism is an ugly philosophy. We find people who live as if they were the center of all being ethically repugnant. We consider them to be narcissists, sociopaths, characterological people who want to be the center of all being and are hence incapable of love. We feel sorry for them, when we aren't being terribly angry at their immaturity (all children are born solipsists and departing from the philosophy is one of the main signs of the advent of real human maturity).
Isn't this a treatise on logic, on knowledge?
Yes, sort of. However, as you can plainly see, logic is flawed and finite. It can never provide knowledge on anything but a provisional basis. It only answers loaded, artificial questions. So we need to find a practical basis for choosing axioms, especially axioms that we eventually add onto the Obvious axioms that nobody sane doesn't really secretly believe13.14.
Solipsism does have one very important virtue. Since it basically takes Descartes' argument and stops right after observing ``I am'' and before the point where Descartes gets into trouble drawing conclusions from that empirical fact and (for better or worse) is completely different from there on, since it is impossible to disprove solipsism rationally, it serves as a lovely tool for helping folks to see that all the other, more sensible-seeming philosophies, are also Bullshit as they can't even logically refute a solipsist.