Wikipedia tells us that:

Solipsism is the philosophical idea that only one’s own mind is sure to exist.

This is obviously dumb and wrong, but philosophers tend to be quite bad at backing up that intuition. Typically, they claim that solipsism is ridiculous or meaningless or has bad consequences (though usually with bigger words), and then dismiss it. Some agree that it is true, say what a great tragedy it is, and then carry on as if it were false for lack of anything better to do. I find this very annoying, because refuting solipsism isn’t actually very difficult if you approach it from the right angle.¹

To better argue against it, I’ll clarify Wikipedia’s definition of solipsism further and break it down into two distinct claims:

  1. One’s mind is sure to exist.
  2. Nothing else is sure to exist.

The first claim dates back to Rene Descartes’s declaration that “cogito, ergo sum;” I think, therefore I am. The cogito argument has been (justly) criticized as relying on unjustified assumptions about individual identity, but we can pare it down to a weaker (and less elegant) argument along the lines of “thinking is occurring, therefore there is a thinking thing.” This isn’t precisely equivalent to that first claim above, but we can reword it as “A mind is sure to exist” without doing too much damage to the philosophy of solipsism; it’s certainly not a clear enough victory to qualify as obliteration, so I’ll turn to the second claim.

I love strong negative claims because they make it very clear what it’ll take to refute them. In this case, if someone can demonstrate that anything other than a single mind exists, solipsism has no choice but to vanish in a puff of logic.² But before I can do that, I should probably nail down the definitions I’m using and get rid of as much ambiguity as I can manage.

So what does it mean for something to exist, then? Most definitions of existence share a common form: something exists if it is present in a specific system. For example, I could say that something exists in the everyday sense if it is present in the system of the physical universe. By this definition, dinosaur fossils exist, but the invisible dragon in my garage does not. More abstractly, I could say that the integer 5 exists in the formal system of arithmetic, but bleem, the integer between 3 and 4, does not. These kinds of existence are not interchangeable, or else I could say that the integer 5 physically exists, and that dinosaur fossils are mathematical symbols.

Unfortunately, any kind of existence that relies on such a system can’t be the existence that solipsism is concerned with, as systems describe the relationships between things. If I have a relationship between two things, then at least one of those things must be something other than the mind I already know about, and my definition would be assuming what I’m trying to prove. Circular logic is a no-no, so the definition of existence I’m looking for must be an absolute one, that allows me to say that a thing either does or does not exist regardless of its relationship to anything else.

But now I’m trapped. If I can’t use relationships to determine existence, then I can no longer say that “thinking is occurring, therefore there is a thinking thing,” because logical implication is a relationship in the system of logic! It is no longer certain that a mind exists, and so solipsism’s first claim is false.

There is no reasonable definition of existence for which solipsism is true; it is a philosophical position which is logically impossible. On the bright side, it provides a thought experiment that’s helpful for clarifying the nature of existence, which is a bad thing to be confused about when trying to answer other philosophical questions.

1: There are philosophers who have made this argument before, but for some unfathomable reason they don’t get very much attention.

2: Credit for this phrase is due to the tragically dead Douglas Adams.