So, solipsism:
Okay, so I’ll just make a fairly simplistic objection here. Solipsism as a position is largely supported by arguments from analogy and arguments from possibility; my focus in this quick post will be to question the viability of the latter.
Implicit (perhaps even explicit) in arguing for solipsism on the basis of possibility is that one can cleanly make a transition from epistemic possibility (i.e “as far as I know, X is possible”) to metaphysical possibility (i.e “X is true in some metaphysically possible world”). But there are some problems here, especially where solipsism is concerned. Now, the sort of trick with solipsism is that there is necessarily no observation or empirical evidence which could refute it. From this, it seems to follow that solipsism is compatible with any *perceived* or conceivable state of affairs. But do you see the problem? This makes solipsism unfalsifiable. Not only is there no empirical basis on which to refute solipsism, there is no empirical evidence to *support* it.
So, my admittedly simplistic objection is this: Given solipsism cannot be substantiated, how can one possibly (pun intended) justify solipsism? That is, how can one move from saying “Solipsism is epistemically possible” to saying “Solipsism is metaphysically possible”, when one has embraced a position which lacks any apparent means by which to make the transition from the former to the latter? I’m more or less using a Moorean shift here. Essentially, I’ve got two competing propositions to consider:
“The external world is just an illusion.”
“My mental experience is at least somewhat accurately representing the world as it is.”
The solipsist is attempting to have me put more trust in certain intuitions that the world could be solipsistic than in my continual experience of the world. This move of rejecting one’s pre-theoretical notions of the world itself is not necessarily problematic (cf. Quantum Mechanics and Special Relativity), but for it to be a rational request, there’s going to need to be some pretty persuasive and powerful evidence presented in defense of doing so.And given the lack of support for or against solipsism (because of its inherent unfalsifiability), it seems, on the face of it, that the latter proposition is justified, while the former proposition lacks *any* means of justification. Notice these objections are not just pragmatic in nature, but that they point (so far as I can tell) to a legitimate epistemic problem with the concept of solipsism.
Mind-Forged Manacles 
I hope you’ll forgive me playing devil’s advocate, but this objection seems to have a fundamental problem.
Can’t the solipsist just turn the objection back around on the realist – so that any kind of empirical observation would similarly count for realism, making it just as unfalsifiable?
And if you can come up with a case where realism would be falsified, would that specific case not then count as evidence for solipsism? Like say “waking up” and having consciousness, but absolutely no sensory input or experience?
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But any empirical observation *doesn’t* count for realism. 😛 The only sort of metaphysical position where it makes sense to say that the world has to “obey” certain logical and/or metaphysical principles is a realist position.
Well, no. One could point out some case where realism would be falsified, but if that case doesn’t actually reflect the way the world is, it’s not evidence for solipsism. Not to mention, solipsism is compatible with *any* perception or conception of the world.
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Well if no observation doesn’t count for realism, then it’s in the same boat as solipsism. Similarly, the solipsist could say that the regularities observed are merely their mind being internally consistent. I doubt you’ll find a solipsist that would say they have control over their perceptions.
As for pointing out a case where realism is falsified that isn’t itself evidence for solipsism, I’d want to see examples of how this could be done. Or frankly any case where realism can be falsified; and if it can’t be falsified, then it seems to fall prey to the same kind of objection you raise here.
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Sorry, what I meant there is that realism isn’t compatible with every perception or conception of the world. But my point concerning consistency is that this really only makes sense under realism. Think about it: Outside of realism, what reason can the solipsist offer to explain the consistency of their experience, and of the particular logical principles that is present in their experience? Sans-realism, there is a greater infinity of ways their perception could be, because to assume otherwise is to assume that some set of principles cannot be “violated”.
No no, what I meant was that even if realism can be falsified, that doesn’t mean realism has been falsified.
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I can’t reply to your last comment directly, so I’m replying here.
My point is more that anything you can say about observations or ability to falsify solipsism can be turned around on realism. I’d like to know in what way realism could be falsified. The entire issue appears to be an unsolvable epistemic black hole, which is why I think apologists like Plantinga made use of it to defend theism.
As for consistency, I’m not sure how this kind of thing is going to be useful in the argument. A solipsist could just as easily appeal to logical necessity as a realist could, and both seem to be in the same kind of bind as to why their perceptions are the way they are, given that it could possibly have been otherwise, given solipsism or realism.
Neither position is trying to explain why things are the way they are, they’re just arguing about what is there ontologically speaking.
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One way it could be falsified is if, for example, uniformitarianism were false. If we made the observation that the universe didn’t behave consistently (in terms of the observed laws of physics), that’d be something very unexpected under realism, and much more plausible under solipsism.
The problem is that “logical necessity” isn’t one thing,because logic isn’t one thing. There are many logics, and the solipsist has no answer for why their perception obeys any particular set of logical principles, and no others. Nor an answer for why the nature of their perception is consistent, for the same reason. 🙂
The problem with solipsism though is that any way the world could conceivably be is compatible with it, so there’s no sensible way to talk about what there is. 😦
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Consider then the idea that the laws of physics could very well change over time. This is certainly an idea that is considered a live possibility in physics. So we could have observations that the laws of physics have changed, or we could infer that indeed they have changed over time to the state we are in now.
As for logical necessity and other logics, how does the realist not face the same issue? The response of “well this is just the way external reality is” can be parroted back in the method of “well that is just the way my mind operates”. What exact explanation can a realist give for the fact that what we consider classical logic holds, other than it just does?
I’m fairly certain that a solipsist could say much the same thing that realism is equally compatible with any conceivable way the world could be, given that it would have to be that way for a consciousness to observe it.
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It seems to me that when we’re trying to decide whether some proposition is metaphysically possible, we don’t start with epistemic possibility and then make some sort of logical move to metaphysical possibility. Instead, we just look at the nature of the proposition itself.
But then again, what is metaphysical possibility anyway? 😛
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Well if we do generally speaking, it doesn’t seem that those saying solipsism is possible are doing so.
Didn’t I answer what metaphysical possibility is in the blog? 😛
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you said: “…metaphysical possibility (i.e “X is true in some metaphysically possible world”).”
That seems circular to me. What’s a metaphysically possible world? Is it different than a logically possible world? (I see those things as identical, but maybe you don’t.)
Anyway, maybe it is the case that actual solipsists aren’t looking at possibility in the same way I described in my previous comment. I’m not actually sure, cause I’ve never met an actual solipsist. But if I was a solipsist, that’s how I would do it.
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What I mean is, something is metaphysically possible if it it is the case in a way the world can be. That is, something is only metaphysically possible if there is some state of affairs where it can happen or be true.
No, I don’t think they’re the same, unless you think there is no “restriction” on the ways the universe can be. 🙂
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Solipsism isn’t really a philosophical theory supported by arguments as much as a problem that falls out of the Cartesian conception of the mind where we are only direct acquainted with our mental states and indirectly know of the external world by virtue of some inference. So the Cartesian conception of mind as an epistemically privileged realm is contrasted with the external world which is only known by representation. Representation introduces the possibility of error, which creates the need for good reasons to think a particular representation is accurate. That opens up the problem of justifying our beliefs about events in the external world, such as our belief that other bodies that resemble ours possess mental states that are like our own.
The threat of solipsism comes in with the assumption that we need to rule it out in some way to justify our belief that others have mental states (part of the infallibilism about knowledge embedded within the Cartesian worldview). If the mental realm is epistemically privileged, and we have to infer the existence of others with mental states, then you need to give arguments for the existence of other minds that resemble the one with which we’re directly acquainted. Things like the argument from analogy become ways for the Cartesian to argue their case. People then criticize the argument because, for example, it’s inductive and has too small of a sample size (one mind attached to one body). Others respond by rejecting the Cartesian conception of mind entirely, like analytic behaviorists.
So there isn’t so much a set of arguments for solipsism as there is a framework from which it appears as a threat to an area of our alleged knowledge about the world.
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Thanks for the input! 🙂
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You should definitely continue writing. I enjoyed this post.
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