Because if 1 is true, then nothing else exists other than their mind. It follows from that, that if a thing is not in their mind it doesn't exist. Therefore they already know (under the assumption of 1), that no other things exist.

They might be wrong about 1, but they obviously cannot be wrong about 1, assuming 1 is true. — Isaac

Your logic makes no sense. Consider, either one of these two scenarios is the case:

1. Only my mind and seven billion other minds and a material universe exist
2. Only my mind and seven billion other minds and a material universe and God exist

Your reasoning is that if 1 is true then nobody can believe that God exists. Obviously that's wrong.

If 1 is true, they cannot believe 2 is true. — Isaac

Of course they can. Why wouldn't they?

1 specifically states that nothing exists other than my mind. So how can I be wrong about the existence of other things under that assumption? I've already declared (by assuming 1), that no other things exist. I can't simultaneously hold a belief that some do (so as to be wrong about that).

I can be wrong about assuming 1, but even without any further data about the rightness or wrongness of 1, I can say that if I assume 1, I can't be wrong about anything else, following from that assumption.

Since I want to retain the possibility of being wrong about things I must reject that assumption. — Isaac

The solipsist doesn't know which of 1 and 2 is true. Your claim that if 1 is true then the solipsist knows that 1 is true is false.

This is just saying that the solipsist could be wrong about solipsism. That's not my argument. My argument is that the solipsist cannot be wrong about anything else, if they are right about solipsism. — Isaac

And that doesn't follow. If the solipsist is right in saying that they cannot know which of 1 and 2 is true then even if 1 is true they are wrong if they believe that 2 is true.

Under 1 it is impossible to be wrong about anything. — Isaac

I think I understand your misunderstanding.

Let's say that a number of coins are hidden in a house. I search the house and find 10 coins. If there are only 10 coins then I know where all the coins are, but I don't know that there are only 10 coins. As far as I know, there may be an 11th coin that is still hidden. Whether or not there is an 11th coin is independent of the 10 coins I have found, even if there are only 10 coins. And I'm wrong if I claim that there is an 11th coin.

So one or more things exist. I know that I exist. If I am the only thing that exists then I know of everything that exists, but I don't know that I am the only thing that exists. As far as I know, there may be something other than me that exists. Whether or not something other than me exists is independent of me, even if I am the only thing that exists. And I'm wrong if I claim that something other than me exists.

Under 1 it is impossible to be wrong about anything. — Isaac

You can be wrong about things other than your mind existing, e.g. God.

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I'm arguing that they cannot be wrong about claims assuming "all that exists is my mind" (or some variation of that).

If they can't be wrong assuming "all that exists is my mind" (or some variation of that), and they want to retain the possibility of being wrong, they must reject the assumption. — Isaac

Again, consider the two scenarios:

1. Only my mind exists
2. Only my mind and God exist

God's (non-)existence is mind-independent. Whether or not God exists has nothing to do with what I believe. As such, I can be wrong. According to you, if I can be wrong – if God's (non-)existence is mind-independent – then something other than my mind exists. Therefore 1 is false and 2 is true. Therefore, if God's (non-)existence is mind-independent then God exists.

Obviously this is fallacious reasoning. Being wrong doesn't depend on the existence of something other than my mind. It is possible that 1 is true and so that God doesn't exist, and so I'd be wrong if I claimed that he did.

Wrong in terms of what ? Your own opinion ? Or something that exceeds us both ? If the latter, you support my point that philosophers as such embrace an externality. — Pie

See here. It's as simple as I can make it.

Their claim that God doesn't exist is lie, not false. They must know their own mind, so they must know whether God is in it or not. — Isaac

They claim that if God exists then he is external to their mind, and they claim that God exists. If he does then they're right, if he doesn't then they're wrong, and they don't know which. It's very simple.

They do know, because if scenario 1 is the case then they cannot be wrong — Isaac

Of course they can. If scenario 1 is the case then God doesn't exist and so their claim that God exists is false.

The solipsist wants to retain being wrong, so they must reject that notion, therefore they do know it cannot be the case that "all that exists is in my mind", it must be one of the alternatives.

The only alternatives are that "something exists outside my mind", or "nothing exists" — Isaac

That's wrong. See here.

If my mind is the only thing that exists then "God exists" is false. If my mind and God are the only things that exist then "God exists" is true. The solipsist doesn't know which of these two scenarios is the case.

To reject 2 either some things which are the case are not in our minds (mind-independence), or nothing is the case (nothing exists. — Isaac

That's a false dichotomy. I've shown that with the example of God's existence. Under 2, whether or not God exists depends on my mind, which is false. But we don't then say that if God's existence depends on the existence of some mind-independent entity then God's non-existence depends on the existence of some mind-independent entity.

God's non-existence doesn't depend on the existence of anything.

They want to be able to be wrong. therefore they must reject 2. — Isaac

They do, that's why they're only an epistemological solipsist. I don't understand what you're trying to argue here.

3. If the solipsist want to retain the ability to be wrong about what is the case then they must reject either 1 or 2. — Isaac

The epistemological solipsist rejects the part that says "all that is the case is in our minds". They only say "all that can be known to exist is in our minds". I've made this clear several times now.

None. As I said, one way out is that nothing exists. I don't think solipsists make that claim though.

Barring that, you must know your own mind. that means you know both what's in it and what isn't Which means you know God isn't so you can't be wrong. If you say "god exists" you're just lying because you already know he's not in your mind - you know what is and isn't in your own mind. — Isaac

Again, this makes no sense. The solipsist can claim that God exists, and that he is wrong if God doesn't exist. Nothing about this entails that the solipsist knows that God exists.

I don't need a mind-independent object to exist for me to be wrong when I claim that God exists. — Michael

We're going round in circles. We've just demonstrated that. — Isaac

No we haven't. We've demonstrated that a mind-independent object (specifically God) needs to exist for me to be right when I claim that God exists.

What mind-independent object needs to exist for me to be wrong? Obviously not God otherwise I wouldn't be wrong. Perhaps a tree? Why must a tree exist for me to be wrong about God existing? Or must God's non-existence exist? That makes no sense at all.

No. It's implausible that if god exists he only exists in my mind therefore if god exists he must do so independently of me. — Isaac

Yes, but this doesn't entail that he knows that God exists.

So simply saying that if mind-independent objects and other minds exist then they do so independently of me doesn't entail that he knows that mind-independent objects and other minds exist.

we also conclude that the only way they can be wrong is if there are mind-independent objects — Isaac

That doesn't follow. I don't need a mind-independent object to exist for me to be wrong when I claim that God exists.

If it is implausible that things only exist in our minds, then we can know that mind-independent objects exist. — Isaac

It is implausible that God exists in my mind, therefore I know that God exists independently of me.

Obviously this is wrong.

So if it is implausible that God is in your mind (because that would mean you couldn't be wrong...etc), the we can know that god cannot be just in your mind. We can rule out that options and so 'know' that what is the case must be one of the remaining options. — Isaac

OK? And the solipsist is wrong he believes that God exists but God doesn't exist. What's the problem?

Because all that exists is their mind, and so God must be in it. — Isaac

The epistemological solipsist claims that we can't know that other minds and mind-independent objects exist. He doesn't claim that other minds and mind-independent objects don't exist. That would be ontological solipsism.

If God is in their mind — Isaac

Why would God be in their mind?

But if the solipsist says "God exists" when God doesn't exist, then they are just lying — Isaac

No, they could honestly believe that God exists. They're just wrong if he doesn't.

But even in this restricted definition, all you've demonstrated is that the solipsist could lie (say "God exists" when in fact God doesn't exist) — Isaac

A falsehood isn't a lie.

I'm asking about the "...it isn't", "...he doesn't", "...it won't" parts. What do any of those propositions mean for a solipsist? How are they any different to the belief in the first place? — Isaac

Because a difference in measure is a plausible account of what it means to be wrong. No one need check, or know that such a difference is the case. It's just that if there were such a difference, you'd be wrong. I'm asking for such an account for the solipsist. — Isaac

I just don't understand your question at all. Consider the simple disquotational account of truth:

"God exists" is true iff God exists.

If the solipsist claims that God exists then he is wrong if God doesn't exist.

I don't understand why you think the solipsist's claim that knowledge of other minds and mind-independent objects is impossible entails that he can't be wrong (or right) about God's existence. There is literally no connection between these positions. So please, help me understand your reasoning, because there is none as far as I can see.

It's not about them knowing. — Isaac

I gave the example of comparing one's own belief to the state of the world as measure of being wrong — Isaac

If it's not about knowing then why are you asking about measures?

But you've not given an account of what it would mean to be wrong for a solipsist. — Isaac

Believing that the Reimann hypothesis is correct, but it isn't. Or believing that God exists, but he doesn't. Or believing that there are other people with private thoughts or sensations, but there aren't. Or believing that the world will end in 10,000 years, but it won't.

If there's no plausible means by which they can be wrong, the rational conclusion is that they can't be wrong. — Isaac

You're not asking for a plausible means by which they can be wrong. You're asking for a plausible means by which they can know that they're wrong. That's not the same thing.

The means by which they can be wrong is just being wrong. If they believe that the Reimann hypothesis is correct, but it isn't, then they're wrong. If they believe that the Reimann hypothesis is not correct, but it is, then they're wrong.

I like how Trump signed a law that increased the penalty for unauthorized removal and retention of classified documents from one to five years.

In fact, your very argument is that private thoughts and sensations have nothing to do with meaning or concepts or whatever, so that there is meaning and concepts and whatever isn’t evidence that there are other things with private thoughts and sensations.

Although I still don't know how you account for the fact that there is the concept of private thoughts and sensations.

Is that an empirical claim ? Or a metaphysical claim ? If it's a metaphysical claim, it's a claim about the concepts knowledge and future, it seems to me. — Pie

I don’t know. Does it matter? I don’t need mind independent objects to exist to have the concept of numbers. I can be the last man alive and yet have the concept of whatever newly mutated monstrous plant emerges from the wasteland. This notion of yours that concepts depend on there being multiple thinking things or mind-independent objects is very wrong.

No. I have never seen a brain, only models, and possibly a piece of meat on a butcher's slab that I failed to recognise. I see your posts, and I assume you speak your mind as I do. I converse with other embodied minds and interact with animal embodied minds. — unenlightened

I think it would be strange to suggest that your mind is (or is inside) my computer screen.

And if there are classified materials involved the president can declassify whatever he wants. — NOS4A2

An ex-President can’t. He’d need to show that he declassified them before he left office.

The FBI is stealing something or planting something, one or the other. — NOS4A2

Or there’s evidence of serious wrongdoing on Trump’s part but wasn’t on the Clintons’ part.

The one used by mathematicians is right. We just ask. — Isaac

There are plenty of unsolved problems in maths, e.g the Reimann hypothesIs. Are you saying that the Reimann hypothesis isn’t truth-apt because it hasn’t been solved? Or does its truth (or falsity) depend on mathematical realism? Or perhaps it’s true (or false) despite mathematicians not having solved it and despite mathematical realism not being the case?

I'm enquiring as to how. If not by some sort of comparison to the right answer, then by what means? — Isaac

How what? How we can know that we’re wrong? Maybe we can’t (a point in favour of skeptical positions like solipsism). But we don’t need to know that we’re wrong to be wrong.

OK, so how do we know if not by comparing the wrong belief to the right one and finding it not to match? — Isaac

How do you know which belief is right so which to use as a comparison? At the moment you seem to be arguing that any knowledge (of maths) is impossible?

I believe so, yes, though I'm far from expert on the matter. But I don't see how formalism rescues the solipsist. — Isaac

The point is that being wrong doesn’t depend on the existence of mind-independent entities. We can get maths wrong even if mathematical realism is false. So the claim that the solipsist’s position that we can’t know that mind-independent objects (or other minds) exist somehow entails that no claim is truth-apt is evidently wrong.

I disagree. The form is similar, but I claim that concepts are special. — Pie

How about “no human can know the future”. That’s true whether I am the only human or one of seven billion. The same with “no human can know that another thinking thing exists”. And the same with “no human can know that mind-independent objects exist”.