Philosophy Bro explains complex ideas of philosophy in easy to understand language, created by Tommy Maranges, the author of Descartes' Meditations, Bro.

Mailbag Monday: Solipsism and Laplace's Demon

Mailbag Monday: A weekly segment that covers readers’ questions and concerns about all things Philosophy, Bro, and Philosophy Bro that don’t quite fit anywhere else. Send your questions to with ‘Mailbag Monday’ in the subject line.

Paul writes,

Hey bro. Love the blog. Can you cover Solipsism for me? Particularly the difference between the metaphysical, epistemological and methodological variants? Thanks bro.

Sure thing, bro. I can absolutely cover solipsism for you. Broadly, solipsism is the idea that the only thing that we know for sure exists is our own mind; that any individual can only know for sure that his own mind exists. Anything else might as well be the Loch Ness Monster, the moon landings, or that one kid’s girlfriend he “met at summer camp. Seriously bro, she’s real. She’s, uh, a model in Sweden.” Sure, Melvin.

Epistemological solipsism pretty much stops right there. What can we say in favor of the world actually existing? How could we possibly prove that anything else is out there? Epistemological solipsism says “I think, therefore I am!” but then stops. Period. End of story. Anything else we want to say is pure conjecture, and unwarranted. Now, sure, if you want to make further assumptions and then talk about what those would entail if they turned out to be true, fuckin’ be my guest. “If gravity does exist, it would work like this.” Cool story, bro. Whatever you say. Aaaanything you now base on gravity is going to be purely hypothetical.

Metaphysical solipsism goes a step further and says, “Not only do I not know that anything else exists, I’m pretty sure nothing does.” Where epistemological solipsism says “you have to admit, it could be true.” Metaphysical solipsism says “Yeah, fuck it. It’s true.” I mean, why even bother with other minds? Even if there’s a ton of shit out there, we would still only have access it through our mind. Not only is your claim that there’s other shit out there unjustified, it’s ridiculous and unnecessary. We have no reason to complicate our ontology - to metaphysical solipsists, believing in other minds is like believing in the tooth fairy. Other, simpler explanations exist.

Methodological solipsism goes in the other direction; it’s the belief that we have to start with solipsism and go from there, but that other things can eventually be known. Since the only thing justified by itself is the belief “I exist,” we absolutely have to start with that. Everything has to follow from that one statement. But methodological solipsists think, “Yeah, 'I exist’, but there’s a ton of shit that follows from that.” Whereas epistemological solipsists think, “Nah. Sorry bro. That’s it. Train stops here.” Descartes started out with something like methodological solipsism, but he pretty quickly, uh, 'proved’ a bunch of other stuff too. The true methodological solipsist thinks that’s the only fucking way to know anything. Everyone has to go through what Descartes did, which is a ton of fucking work, if you ask me.

Is solipsism feasible? Well, it’s hard to say. David Lewis believed that if believing in a ton of unprovable shit is philosophically useful, we should fucking go for it - metaphysical solipsists are on the opposite end of the spectrum. Of course it would be useful as shit if we could prove that other minds existed or the external world is real. But we can’t, so it doesn’t matter how much power it would give us - it’s all unsupportable.

Brotrand Russell famously thought solipsism was so useless, it literally fucking paralyzed its adherents - if anyone was a functioning solipsist, it meant that they weren’t actually solipsists. After all, the idea of telling someone you’re a solipsist, of defending that idea, seems pretty fucking ridiculous when you think about it. “Hey, thing I made up in my mind who doesn’t really exist! Guess what other thing I made up in my mind: solipsism!” It’s like introducing two imaginary friends, except one of them goes around telling you he’s imaginary, so why bother? Solipsism is lonely, bro; don’t judge.

So if someone tells you he’s a solipsist, either he thinks you’re as imaginary as the case for Intelligent Design, or else he doesn’t really believe that you don’t exist. After all, for as many solipsists as there are, a maximum of one of them can be right.

-Ž writes,

Love your site. I’d love it even more if you did an analysis on Laplace’s demon. Just a suggestion.

Of course, dude, Broplace’s (pronounced BRO-plas or LUH-plas if you’re lame) Demon. Laplace was a French astronomer back in the day who believed strongly in the determinism of physical laws - that the state of the universe combined with the laws uniquely predicted one possible future - the laws of nature say exactly how the universe will unfold. He proposed a thought experiment that ran a little something like this:

Let’s say there was a demon who knew all the laws, and could see exactly how everything was, and was kind of a dick. Weird, right? He can see everything? Except he wouldn’t even need to watch you shower, because he could predict exactly how and in what way you would do it - he would know which way every drop of water would bounce. He would be able to calculate, decades in advance, who you will marry, whether her mom will hate you, and when you’ll die. He could even figure out who wins the WNBA championship this year if he cared enough to do the calculations (he doesn’t). Why? Because it’s all governed by physics, and physics is predictable as shit, bro.

Yeah, that seems scary as shit. Of course, at first glance, it’s not about a literal demon; that’s just the metaphor for saying, “Bro, everything is predictable with physics.” And if determinism is true, then that seems to be the case.

Except here’s the thing: 'determined’ is not the same as 'predictable’. For starters, it’s impossible for something, or someone, within a system to predict with perfect accuracy the outcome of that system. Why? Imagine a giant fucking supercomputer simulating a day into the future. One day. Except the computer would have to take into account its own simulation, since it is inside the system; to figure out those effects, it would have to run the simulation entirely within the simulation. And of course, in that simulation, he’ll have to run it again… and so on. It just doesn’t work. And it turns out that’s true even for a broputer with infinite broputing power.

So it looks like for Laplace’s Demon to really mean anything, it actually does have to be outside the system. Since we’re inside the system, all the Demon thought experiment tells us is that we can approximate future events using physics, something we can do even if determinism isn’t true. Some have suggested that it also means that free will is an illusion, since the demon would know what choice you’d make way the fuck before you actually made it; attentive brotégés know that’s not necessarily the case.

See Russell’s critiques of solipsism, as well as his thoughts on a range of topics including religion and metaphysics in general: Mysticism and Logic: And Other Essays (1918)

Wikipedia has a page on solipsism that offers brief descriptions of the three varieties. It also has a page on Laplace’s Demon.

Papers by real-life solipsists are hard to come by, it turns out, for the reasons discussed above.

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