Why Not Solipsism?

You can't be certain, but I'm probably real (sorry)

If you were a weird kid, Solipsism was probably one of the first philosophical views you encountered. Not because you spent any time reading about it - you probably just had a weird “Ahah” moment where you realised the only thing you were certain you had direct experience of was your mind, and thought “Wait, what if I’m the only one here?”. Then you tried not to think about it, and went back to eating your fruit roll up. At least, that’s what it was like for me.

Solipsism is often bushed aside as an immature philosophical view, but it’s hard to pin down what exactly about it is objectionable. On the face of it, it almost seems common sense. You’re only believing in the part you’re sure of (you) and denying the existence of (or being agnostic about) all the other stuff. It’s seems epistemically conservative.

However, I think we have good reasons to think it’s false. Well, I’m actually certain it’s false for you because I know my mind exists - although, of course I would say that. I could just be a very convincing part of your hallucination oooOOOOOOoooOOOOOooooo send nuUUUuudes oOOOOOooooOOOOooo.

Extraordinary Luck

If you read Bentham’s Bulldog (and knowing who we appeal to, that’s probably a lot of you nerds) you’ve probably read a lot about anthropics - specifically the Self Indication Assumption. SIA is a theory of anthropics from Nick Bostrom that says you “should reason as though you are randomly selected from the set of all possible observers”.

For those who haven’t read Matthew’s 26784 articles on the topic, here’s an example to illustrate. Imagine a coin is flipped. If it lands on heads, 1 person comes into existence. If it lands tails, 10,000 people are brought into existence. All you know is the above, and that you were brought into existence. What chance do you think the coin came up tails?

SIA says you should think tails was 10,000x more likely than heads, and this strikes me as very plausible. The scenario where tails came up predicts your existence way more than the timeline where heads came up. If it was heads, you’d have to be very lucky to be the one possible person to be brought into existence (or unlucky, I guess, depending on how sad you are).

So, how does this apply to Solipsism? Well, if SIA is correct, then you should think the chances of you existing is way higher in a world with other minds than a world with a single mind. It would be awfully convenient if only one mind exists and it happened to be you. Whereas your existence is more expected if there’s an enormous amount of people, and guaranteed if there’s infinite people (which there might be!).

A Surprising World

Your existence isn’t the only thing that would be surprising if Solipsism is true, the contents of your consciousness are pretty strange too. If your mind is all there is, then it follows that it’s not subject to any psychophysical laws. After all, there is no physical realm for your mind to interact with. So, it seems like if Solipsism is true, your mind could have had literally any sort of content.

You could’ve had a psychedelic experience. You could’ve experienced nothing but the sensation of warm bath water. You could’ve experienced the Bud Light “Wazzup” ad on loop forever. In fact, it seems like in most possible conscious experiences there aren’t a bunch of people walking around that walk and talk like you. The probability space taken up by conscious experiences that look like they have other people in is tiny. So, just as in the previous section, this datum is awfully surprising if you’re the only one here.

However, if other people can exist, it seems way more likely that you wouldn’t seem alone in your consciousness. This is because you’d be produced by the same mechanism that produces all the other minds, and you’d likely be close to them geographically (unfortunately).

Now, maybe we can adopt a sort of Solipsism that says the physical world does exist, but only you have a mind. Everyone else is just a philosophical zombie. However, again, it would be awfully weird if the mechanism that brought humans about produced all of them the same way, with the same sort of brain activity, and yet only one of them was conscious. Just one mind out of the nearly 100 billion humans who have ever lived? That seems astronomically unlikely, and even more so if we include animals.

A Note on Certainty

It’s at this point that the Solipsist might say “Ahah! You haven’t been able to prove that Solipsism is false! You’ve only provided probabilistic arguments! I guess we’ll never know”. However, I think certainty is way too high a standard for knowledge, and commits you to saying that you don’t know a great many things that you probably do know. It’s irrational to withhold a belief simply because you can’t be 100% certain. Otherwise, I’d be committed to saying I’m not sure if dogs exist simply because I can’t be certain they didn’t all pop out of existence 2 seconds ago - which is crazy, because I totally know that dogs exist (I am very smart that way). At the very least, it’s rational to believe that dogs exist, just as it’s rational to believe that Solipsism is false. Sorry, no walking around naked in public for you.