To me the tricky part is that the solipsist is making claims about any rational agent, existence or not. — Pie

Yes. It’s no different to saying “no human has two heads” which is true if I’m the only human or if I’m one of seven billion.

So against what measure am I comparing my belief that the square root of four is one, in order that it is wrong? — Isaac

There is no measure. It’s just either right or wrong.

Do you understand the difference between mathematical realism and mathematical formalism? The solipsist says that we cannot know that mind-independent mathematical entities (as per realism) exist. He doesn’t say that we cannot get string manipulations wrong (as per formalism).

I believe the square root of four is one. But it isn't, it's really two.

The belief is in my mind.

Where's the real fact? — Isaac

It’s not anywhere. I reject mathematical Platonism.

I can't prevent you from softening the meaning of 'external' to make your case. I just think that you simultaneously diminish the significance of the claim, as if the solipsist is merely denying a particular metaphysics of the external world (such as Epicurean atomism, or the 'reality' of everyday objects) --- as opposed to the external world itself. — Pie

But that “soft” externality is the kind of externality that the solipsist denies can be known. They don’t deny knowledge of the metaphorical externality that you apply to such things as maths. The solipsist accepts that we can get maths wrong.

Can one be wrong about the square root of 2 ? — Pie

Yes. And one can be wrong about the square root of two even if one cannot know that other minds and mind-independent objects exist. Which is precisely why it is wrong to say that epistemological solipsism entails that no claim is truth-apt.

Why do you assume that the external is an object? — Pie

Because that's the kind of external thing that the solipsist says cannot be known to exist. If you mean something else then you're not addressing the solipsist's claim.

It's just a spatial metaphor. — Pie

Could you explain it without using a spatial metaphor?

I claim that that concept itself is either external or not worth talking about. — Pie

I have no idea what it means to say that a concept is external. Is the concept of a horse something that I can encounter and pick up?

What is it that we could be wrong about, according to the skeptic, who makes an assertion about us ? — Pie

Any thinking thing, whether there be just one thinking thing or two thinking things or seven billion thinking things.

Solipsism is a skeptical position, and skepticism is the position that we could be wrong. That you are somehow turning it into the position that we can't be wrong should show you how mistaken you are.

How can I make a statement that's true or false without something I can be right or wrong about ? — Pie

I can be wrong about things that don't exist. If I claim that God exists then my claim is false if God doesn't exist. If I claim that you have private thoughts and sensations then my claim is false if you don't have private thoughts and sensations. If I claim that mind-independent objects exist then my claim is false if mind-independent objects don't exist.

if an epistemological solipsist, that it's wrong to assume we could be wrong — Pie

If you're going to continue to argue against this strawman then there's no point in me continuing. The epistemological solipsist doesn't claim that no claim is truth-apt. How many times do I have to explain this?

Trump real estate appraiser hands over thousands of documents to N.Y. AG in civil probe

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He’s having a bad day.

It's impossible according to one of our best empirical theories. — Pie

And the same could be true of solipsism. It is an empirical fact that knowledge of other minds is impossible, in the same way that it is an empirical fact that knowledge of events outside our light cone is impossible. Or it could be that knowledge of other minds is only impossible in practice, since we lack the technology to detect mental phenomena.

I'm not saying that knowledge of other minds is grammatically impossible. I don't even know what this means. Whereas you seem to be trying to argue that there being other minds is grammatically necessary, which is absurd. You can't define things into existence. It doesn't work with God and it doesn't work here either.

The issue is whether you take the essence of mind to be so radically immaterial and apart from the causal nexus that it's impossible, even in principle, to discover that number. — Pie

It's impossible in principle to discover what is happening outside our light cone, but (at least according to the realist) stuff is happening outside our light cone.

But is it possible for timetravelling scientists from Neptune to figure it out with scanners ? With 99.875% accuracy over 10000 trials ? — Pie

I don't know. That depends on how the hard problem of consciousness is solved. If your only argument against solipsism is that some hypothetical people with a sufficiently advanced technology might have the means to detect either mental phenomena or the physical phenomena which necessarily gives rise to mental phenomena then this doesn't refute the claim that you, right now, cannot know what number I am thinking of, or that I am thinking of a number at all. But I still might be thinking of a number.

I can think of a number and not tell you or anyone about it. It is impossible for you to know what number I am thinking of, or even that I am thinking of a number.

I don't know what else to tell you. This really is a self-evident fact. If your understanding of language and the world denies this very fact then your understanding of language and the world is wrong.

What would you count as evidence for a private thought ?

Is a private thought just something we might quietly 'say' to ourselves ? Is it something that, in principle, could be detected and translated by a sufficiently advanced neuroscientist from Venus ?

Or is it on another 'plane' entirely, untouched and untouchable by the 'non-mental'? — Pie

I don't know, hence the hard problem of consciousness.

If you leap from the boring, typical talk of private thoughts to the 'official theory' of the ghost, then the only difference between a P-Zombie and a real boy is ... nothing at all. — Pie

The difference is that the p-zombie doesn't have the private thoughts. You keep switching between accepting that there are such things to then not? I don't understand it.

We must pretend to admit that possibly that Hitler actually loved the Jews. Or that the guy who beats and rapes his wife 'actually' loves her, because the truth is behind or other than any evidence we can summon for this or that judgment. — Pie

Yes, it is possible. But a case can be made that it is incredibly unlikely because one's private thoughts are what motivate behaviour. So as I said before (either in this or another thread), the only suitable argument against solipsism is to claim that something without a mind wouldn't behave in this way. Trying to argue against the coherency of solipsism is just wrong.

The Routledge article also says:

Second, realists hold that moral facts are independent of any beliefs or thoughts we might have about them. What is right is not determined by what I or anybody else thinks is right. It is not even determined by what we all think is right, even if we could be got to agree. We cannot make actions right by agreeing that they are, any more than we can make bombs safe by agreeing that they are.

Traditionally, as I understand it at least, the moral realist's claim that murder is immoral isn't comparable to the claim that murder is illegal. They tend to make a more substantive claim than that. They think that murder has some (natural or non-natural) moral property that is then correctly (or incorrectly) described when we claim that murder is immoral.

But if you want to argue for a more minimal account of moral realism (that moral claims are truth-apt and some are true) then I suppose you're welcome to, although I don't suspect that's the kind that the OP is asking about. I suspect he’s asking about the meta-ethics that is comparable to mathematical realism, whereas yours is comparable to something like mathematical formalism, which is a type of mathematical antirealism.

Both play a role in inferences. Both have meaning. I don't have to know your private thoughts to reason about private thoughts in general. The norms for their application are not hidden. — Pie

The norms for the application of the phrases aren't hidden but the referents are hidden. I can talk about the future and your private thoughts but it is impossible for me to know what will happen in the future and what your private thoughts are.

So, again, what's the problem?

It plays a role in inferences. — Pie

I don't understand what you mean. The phrases "the future" and "your private thoughts" each refer to something that is necessarily inaccessible to me. What's the problem?

Pray tell how we might evaluate from the outside whether Harry loved Sally, having met her? — Pie

We can't, hence skepticism. Knowledge of private matters is impossible.

Or how 'love' could have a public meaning if its referent is infinitely private. — Pie

I don't know how it does. How does the word "future" refer to something that is necessarily inaccessible? We're just clever people that are somehow able to do clever things with language.

Is it not that certain statements about the speed of life are objective ? — Pie

The realist will say that there is more to the world than just statements. There is the statement "the cat is on the mat" and there is the cat being on the mat. The latter is the case even if nobody talks about it and is what makes the former true.

Do they? — Isaac

At least as I have always understood it, with ethical naturalism and ethical non-naturalism being the two main types.

You said moral realists believe morality relates to objective facts. Being part of a group of behaviours associated with a particular word is an objective fact. If you mean to claim some additional criteria for moral realism, then state it. — Isaac

Moral realists claim that moral facts are objective in the sense that the speed of light and the existence of Mercury are objective. They don't claim that they are objective in the sense that the legality of marrying a 16 year old is objective. This latter kind of "objectivity" would count as a type of relativism. In your example, the morality of an action is relative to and determined by the linguistic practices of a language community.

The meaning of a word in a language is objective. We don't all have our own personal meanings, we couldn't talk if that were the case. — Isaac

That has no bearing on what moral realists mean.

Moral realists claim that there are objective moral facts. Your account makes for morality to be a linguistic convention which would be a kind of moral relativism.

Language?

Shoplifting is wrong because it's the sort of thing we use the word 'wrong' for. — Isaac

This wouldn’t be moral realism though.

Incidentally, for a similar reason I think there’s a fundamental problem with Putnam’s brain-in-a-vat hypothesis and his causal theory of reference. I think it implausible that the brain-in-a-vat cannot refer to itself as being a brain-in-a-vat. We’re intelligent creatures and we’re able to talk about things beyond what is given in experIence. I admit that I don’t know how we do it but I think it evident that we do.

If anything his argument is a refutation of the causal theory of reference rather than its intended target of metaphysical realism.

Given that you seem to understand what I mean when I say “no, because I think and feel and see things that I never talk about or act on. I dream, I imagine, I lie, I ignore, etc.” and given that you seem to understand the conceptual difference between a genuine loving relationship and a convincing act, why wouldn’t your “semantic” contemplation lead you to agree with the sensibility of my position?

What I think is actually going on is that you’ve been convinced by something like Wittgenstein’s account of how language is learned and that you think such an account of learning doesn’t work with my position. Which I think should just show the limitations of Wittgenstein’s account, or at least with your interpretation of its implications.

Is it not better to say that we are constituted by all we do and say ? — Pie

No, because I think and feel and see things that I never talk about or act on. I dream, I imagine, I lie, I ignore, etc.

And I assume that there are others to whom this is also the case. Although the solipsist will argue that I cannot know this.

I totally get that, but that's a tangent, an exception. — Pie

And by an exception you mean a coherent example of there being a private mind behind the public expression that cannot be known? Then what left is there to say? The case is proved.

Do you think that things must be proved in order to be true?

Or are there things that are true yet unproven? — Banno

I believe he’s saying that if you can’t prove that aliens exist then you don’t know that aliens exist, even if you believe that aliens exist and even if aliens exist. The same with there being other minds.

What, sir, is this mysterious X that separates the convincing P-zombie from the genuine article? — Pie

The private thoughts and feelings and perceptual sensations that might go unexpressed, or that can be contrary to the expression (i.e in the case of lying).

Again consider the example of a genuine loving relationship compared to a convincing act.

Of course it matters, in the ordinary lingo. We can both speak with the vulgar and think with the wise.
Note please that what people think is still linguistic.

Perhaps 'feeling is first,' but justification is going to involve reports or ascription of feelings, typically used as the premises or conclusions of inferences. 'John's mad because Sally lost the car keys again.' — Pie

The point is that the thinking, feeling part is private. Such thoughts and feelings might be expressed, but the private thoughts and feelings nonetheless exist and are prior to the public expression.

And given the concept of lying, we cannot know that someone's public expressions are an accurate representation of their private thoughts and feelings. We often assume it, and might even often be correct, but skepticism is warranted all the same. From the understanding that we cannot know what someone's private thoughts and feelings are there can then be the understanding that we cannot know that someone has private thoughts and feelings. They might just be a philosophical zombie, engaging in the same public behaviour and making the same public expressions as a thinking, feeling person.

The 'important' part of my mind, as I see it, is the thinking, linguistic part. — Pie

So whether or not someone is lying or being honest isn't important? It doesn't matter what they think or feel, only what they say and do? That seems quite psychopathic if I'm being honest. I don't just want my partner to "go through the motions" of a loving relationship.

'My' version of green doesn't matter, but my use of 'green' does. Even my secret use of 'green' in private inferences is manifest eventually in public assertions the way I react to others' talk. — Pie

Doesn't matter to what? To the practicalities of everyday life? Sure. But the philosophical questions regarding perception and ethics and epistemology and realism and so on can still be worth discussing, and likely have true and false answers.

At the moment your position amounts to saying that solipsism might be true but doesn't affect how I (or we) live.

Why did you ask me why I asked you what it means to exist if existence isn't a public concept? — Pie

I asked you because the inner workings of your mind are private and I need to you publicly express them.

So why did you ask me?

If no one is wrong, no one is right. There wouldn't be misunderstanding. Just screeching primates who could no longer coordinate their doings in the world.

Concepts need not be perfectly definite. Roughly speaking, they are patterns in what we do. We perform concepts. Away from practical life, performances are less rote. Tentative, exploratory uses compete for wider assimilation. — Pie

I don't quite know what you mean here (quite nicely proving my point).

What I'm trying to get at is this: let's say that you and I agree to meet up at the gym. I then have to cancel. We then never realize that we misunderstood which gym we were to meet up at; I thought the one on the east side of town, you thought the one on the west.

There's stuff that goes on in our head that is never made public.

Central to the solpisism subissue here is that of whether concepts are public or private. I claim it's incoherent to say they are not public. — Pie

If that were the case then we wouldn't have to ask people what they mean. We wouldn't have to ask them to clarify their position. There wouldn't be misunderstanding. Why did you ask me what it means to exist if existence is a public concept?

Anyway, all I have been trying to do here is explain that the claim "it is impossible to know that other minds and mind-independent objects exist" is different to the claim "no proposition is truth-apt". The solipsist claims the former, not the latter, contrary to @Pie's misrepresentation.

What hypocrisy? I don't see why the non-solipsist would not "demand it of himself", or admit to a known shared understanding of the word "exist". But if the solipsist admits to it, then...see my previous post. — Luke

The non-solipsist says "it is possible to know that other minds and mind-independent objects exist".
The solipsist replies with "it is impossible to know that other minds and mind-independent objects exist".
The non-solipsist then says "but what does it mean to exist"?

Why is the non-solipsist asking that? Presumably he knows, or has some notion of, what it means to exist which is why he claims that it is possible to know that other minds and mind-independent objects exist.

The 'we' is 'deeper' or more 'primordial' than the (linguistic) 'I.' — Pie

I doubt that. If we look to non-human animals, and perhaps babies and people with certain developmental disabilities like autism, I suspect that they have a greater difficulty in understanding that other animals/people have minds and thoughts and feelings like their own. The notion of "other minds" requires a degree of inference that comes after self-recognition. To understand that other people have minds you must first understand that you have a mind.