And yet you said:

You, in that world, might not know, in that world, that all that exists is your mind — Isaac

And as I mentioned above, there's more to knowledge than just knowledge of what exists. In such a world I don't know the square root of two, I don't know what I would have seen had I chosen a different course of action, I don't know what I will feel tomorrow, etc.

You are conflating "if Y exists then X knows that Y exists" and "X knows everything". These are not the same thing. The former would be true, the latter not.

In such a world I don't know the square root of two, I don't know what I would have seen had I chosen a different course of action, I don't know what I will feel tomorrow, etc. — Michael

I disagree. In a world where all there is is your mind, there's no uncertainty. You know exactly what the square root of two is, because it's available directly to your conscious mind. Same with what you'll feel tomorrow.

There can be no source of uncertainty other than from some state external to the system carrying out the inference.

You know exactly what the square root of two is, because it's available directly to your conscious mind. — Isaac

No it isn't.

Same with what you'll feel tomorrow.

And again, no.

Well. I'm sold. Super convincing argument. Well done. Have you considered a career in politics?

Well. I'm sold. Super convincing argument. Well done. Have you considered a career in politics? — Isaac

I could say the same about your bare assertion that the square root of 2 and my future feelings would be directly available to my mind.

You, in that world, might not know, in that world, that all that exists is your mind — Isaac

And again, you refuted your own argument here. I don't know that all that exists is my mind. This, in fact, is epistemological solipsism.

I could say the same about your bare assertion that the square root of 2 and my future feelings would be directly available to my mind. — Michael

It's not a bare assertion. I followed it with the argument...

because it's available directly to your conscious mind...

There can be no source of uncertainty other than from some state external to the system carrying out the inference. — Isaac

you refuted your own argument here. — Michael

My argument is about the version of us doing the assessment about the feasibility of those possible worlds.

There can be no source of uncertainty other than from some state external to the system carrying out the inference. — Isaac

The square root of two is mind-independent even if only my mind exists. You conflate mind-independence with mind-independent existence. The solipsist rejects (knowledge of) mind-independent existence.

And there's uncertainty because of the logical argument I gave above:

1. Bp (premise)
2. ¬□p (premise)
3. Bp ∧ ◇¬p (from 1 and 2)

The soundness of this argument doesn't depend on something other than my mind existing.

My argument is about the version of us doing the assessment about the feasibility of those possible worlds. — Isaac

And yet you admitted that if only X's mind exists then X doesn't know that only his mind exists. I don't know what else I can tell you; you just admitted to epistemological solipsism.

The square root of two is mind-independent even if only my mind exists. — Michael

Just stating it doesn't help. Look...

The square root of two isn't mind-independent even if only my mind exists.

Did that help?

you admitted that if only X's mind exists then X doesn't know that only his mind exists. I don't know what else I can tell you; you just admitted to epistemological solipsism. — Michael

No, because the epistemic solipsists can further analyse what it would mean for X (that even if X didn't know they know, they would, in fact, know) and thereby need to reject the option. Having found they need to reject the option, they cannot coherently claim to also not know if it's true.

I'm familiar with Kant's version, and I have little argument with it; but I don't think he has the same thing in mind that you do. — Janus

I must agree. And....good series of comments.

No, because the epistemic solipsists can further analyse what it would mean for X (that even if X didn't know they know, they would, in fact, know) and thereby need to reject the option. Having found they need to reject the option, they cannot coherently claim to also not know if it's true. — Isaac

Your reasoning is faulty.

1. If Y exists then X knows that Y exists
2. X knows that if Y exists then X knows that Y exists

1 does not entail 2. And nor does it entail any of these:

3. X knows that he will be happy tomorrow
4. X knows that the Riemann hypothesis is true
5. X knows that the Riemann hypothesis is false
6. X knows that had he chosen some other course of action then he would have been sad
7. X knows that God doesn't exist

I don't know how much simpler to explain this to you so if you can't understand this then we're never going to progress.

1. if Y exists then X knows that Y exists
2. X knows that if Y exists then X knows that Y exists

1 does not entail 2. — Michael

Indeed. It's not a rendering of my argument.

1. render all that is the case as the set {Y,Y,Y,Y....} for all Ys
2. if Y exists then X knows all properties of Y (a consequence of all Ys being in X's mind)
3. X therefore cannot ever be wrong about what is the case (since what is the case is entirely constituted of all the Ys)
4. Z (our epistemic solipsist) entertains a possible world in which 2 is true.
5. Z then has to admit that in that possible world X cannot be wrong about what is the case (whether X knows this or not is irrelevant)
6. no X is ever in a situation where they cannot be wrong about what is the case.

5 and 6 are a contradiction. Z has to either reject 5 or reject 6.

Maintaining 6, Z rejects 5.

If Z rejects 5, they cannot also coherently claim scepticism about whether 5 is the case or not.

5. Z then has to admit that in that possible world X cannot be wrong about what is the case (whether X knows this or not is irrelevant) — Isaac

You're equivocating. As per 1 and 2, "what is the case" is restricted to "Y exists". Hence you are only concluding:

If Y exists then X knows that Y exists

God's non-existence, the Reimann hypothesis, and being happy tomorrow aren't one of the Ys defined in 1 and assumed to exist in 2 (what would it even mean to say that God's non-existence exists?).

rationality mandates only validity, not soundness once the boundaries of the empirical realm have been crossed. — Janus

How are such boundaries established ?

I am open to criticism, but any criticism will be parsed through my own critical filter and accepted or rejected depending on how I judge its plausibility. — Janus

:up:

Indeed, and we want that filter to be a good one ! Surely you update it in terms of what it lets through. Your 'I'-filter isn't static.

"The internal"; my thoughts, feelings and bodily sensations are what is most immediately present to me. Your thoughts, feelings and sensations are present to me only insofar as they can be conveyed by what you say about them: if you are being honest. If you are lying or hiding your thoughts and feelings then I may have no idea; unless I feel your actions are giving you away, and I could be wrong about that. None of this has anything to do with Descartes. — Janus

From Wiki:

Subjectivism is a label used to denote the philosophical tenet that "our own mental activity is the only unquestionable fact of our experience." The success of this position is historically attributed to Descartes and his methodic doubt.

As a reader of Heidegger, you may find this relevant. (I prefer it from the horse's mouth, but I'm not going to type up piles of prose for something so established.)

Descartes stands at the beginning of modern philosophy and Heidegger accepts Descartes' role in the history of metaphysics. Descartes is the first thinker who discovers the "cogito sum" as an indubitable and the most certain foundation and thereby liberates philosophy from theology. He is the first subjectivistic thinker in the modern philosophy and he grounds his subjectivity on his epistemology.

The orientation of the philosophical problems with Descartes starts from the "ego" (the "subject") because in the modern philosophy the "subject" is given to the knower first and as the only certain thing, i.e., the only "subject" is accessible immediately and certainly. For Descartes, the "subject" (the "ego", the "I", "res cogitans") is something that thinks, i.e., something that represents, perceives, judges, agrees, disagrees, loves, hates, strives, and likes. "Descartes calls all these modes of behavior cogitationes." (1) Therefore, "ego" is something that has these cogitationes. However, the cogitationes always belongs to the "I", I judge, I represent, etc. Heidegger maintains that Descartes' definition of "res cogitans" says to us that "res cogitans" is a res whose realities are representations.
...
Starting with Descartes, the subject becomes the center, and the subject, as the first true being, has priority over all other beings. Contrary to this priority of the subject, Heidegger's goal is to show that there is no subject distinct from the external world of things, because Dasein is essentially Being-in-the-world. Therefore, Heidegger puts together the separation of the subject and the object by the concept of "Dasein" which is essentially a Being-in-the-world. However, Being-in-the-world does not mean that it is like a piece of chalk in the chalk box. Being-in, as the most essential and existential characteristics of Dasein, signifies the expression of such terms as "dwelling," "being familiar with," and "being present to."

The distinction between the subject and the object makes the possibility of the distinction between the knower and what he knows.

https://www.bu.edu/wcp/Papers/Cont/ContCuce.htm

@Janus
Here are some Dreyfus-translated passages from Heidegger (from Being-in-the-World). I recommend especially the chapter "The Who of Everyday Dasein."

While it's beyond the minimal epistemic given to just assert these things, I read them as gesturing toward something similar, toward our being-in-a-world-in-language-together as our basic human situation. I am not some ghost viewing a spectacle. That's an example of 'interpretedness,' which is to say historical baggage, taken for granted unquestioningly.
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The one as that which forms everyday being-with-one-another...constitutes what we call the public in the strict sense of the word. It implies that the world is always already primarily given as the common world. It is not the case on the one hand there are first individual subjects which at any given time have their own world; and that the task would then arise of putting together, by virtue of some sort of arrangement, the various particular worlds of individuals and of agreeing how one would have a common world. This is how philosophers imagine these things when they ask about the constitution of the inter-subjective world. We say instead that the first thing that is given is the common world -- the one.
...
Dasein, in so far as it is, has always submitted itself already to a 'world' which it encounters, and this submission belongs essentially to its being.
...
Primarily Dasein is 'one,' and remains so.
...
As a "one's self," the particular Dasein has been dispersed into the 'one' and must first find itself.
...
"[The one] is the 'realest subject' of everydayness."

"The internal"; my thoughts, feelings and bodily sensations are what is most immediately present to me. ... None of this has anything to do with Descartes. — Janus

Descartes famously emphasized that subjective reality is better known than objective reality, but knowledge of the objective reality of one’s own existence as a non-physical thinking thing is nearly as basic, or perhaps as basic, as one’s knowledge of the subjective reality of one’s own thinking. For Descartes, knowledge seems to start with immediate, indubitable knowledge of one’s subjective states and proceeds to knowledge of one’s objective existence as a thinking thing. Cogito, ergo sum (usually translated as “I think, therefore I am”) expresses this knowledge. All knowledge of realities other than oneself ultimately rests on this immediate knowledge of one’s own existence as a thinking thing.

From the horse's mouth (and he is/was a genius) :

Now that I have convinced myself that there is nothing in the world – no sky, no earth, no minds, no bodies – does it follow that I don’t exist either? No it does not follow; for if I convinced myself of something then I certainly existed.

But there is a supremely powerful and cunning deceiver who deliberately deceives me all the time! Even then, if he is deceiving me I undoubtedly exist: let him deceive me all he can, he will never bring it about that I am nothing while I think I am something. So after thoroughly thinking the matter through I conclude that this proposition, I am, I exist, must be true whenever I assert it or think it.

But this ‘I’ that must exist – I still don’t properly understand what it is; so I am at risk of confusing it with something else, thereby falling into error in the very item of knowledge that I maintain is the most certain and obvious of all. To get straight about what this ‘I’ is, I shall go back and think some more about what I believed myself to be before I started this meditation.

Well, then, what did I think I was? A man. But what is a man? Shall I say ‘a rational animal'? No; for then I should have to ask what an animal is, and what rationality is – each question would lead me on to other still harder ones, and this would take more time than I can spare.
...
Since now I am pretending that I don’t have a body, these are mere fictions. Sense-perception? One needs a body in order to perceive; and, besides, when dreaming I have seemed to perceive through the senses many things that I later realized I had not perceived in that way.
...
Thinking? At last I have discovered it – thought! This is the one thing that can’t be separated from me. I am, I exist – that is certain. But for how long? For as long as I am thinking. But perhaps no longer than that; for it might be that if I stopped thinking I would stop existing; and I have to treat that possibility as though it were actual, because my present policy is to reject everything that isn’t necessarily true. Strictly speaking, then, I am simply a thing that thinks – a mind, or intelligence, or intellect, or reason, these being words whose meaning I have only just come to know. Still, I am a real, existing thing. What kind of a thing? I have answered that: a thinking thing.

Well, then, what am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, wants, refuses, and also imagines and senses.

Isn’t it one and the same ‘I’ who now doubts almost everything, understands some things, affirms this one thing – namely, that I exist and think, denies everything else, wants to know more, refuses to be deceived, imagines many things involuntarily, and is aware of others that seem to come from the senses? Isn’t all this just as true as the fact that I exist, even if I am in a perpetual dream, and even if my creator is doing his best to deceive me? These activities are all aspects of my thinking, and are all inseparable from myself. The fact that it is I who doubt and understand and want is so obvious that I can’t see how to make it any clearer. But the ‘I’ who imagines is also this same ‘I’. For even if (as I am pretending) none of the things that I imagine really exist, I really do imagine them, and this is part of my thinking. Lastly, it is also this same ‘I’ who senses, or is aware of bodily things seemingly through the senses. Because I may be dreaming, I can’t say for sure that I now see the flames, hear the wood crackling, and feel the heat of the fire; but I certainly seem to see, to hear, and to be warmed. This cannot be false; what is called ‘sensing’ is strictly just this seeming, and when ‘sensing’ is understood in this restricted sense of the word it too is simply thinking. — Descartes

This is basically a 'speculative' theological vision. Descartes 'is' 'thinking' or language. It's our being jointly 'enworldled' and 'enlanguaged' that is 'given.' Descartes and Kant seemed to have got tangled up in the 'I' and its social function. 'I think' is attached to 'P' within a normative, scorekeeping game. But 'P' is is,in its intelligibility even to its user, prior to that user, deeper than that user. Descartes implicitly asserts the autonomy of reason by striving toward presuppositionlessness. So I claim, and so I claim Hegel and Heidegger claim, to name just two. The assault on the Cartesian subject is, as I understand it, one the big events in relatively recent philosophy.

Taken as an intelligible (geistig) or an abstract being, that is, regarded neither as human nor as sensuous, but rather as one that is an object for and accessible only to reason or intelligence, God qua God is nothing but the essence of reason itself. But, basing themselves rather on imagination, ordinary theology and Theism regard him as an independent being existing separately from reason. Under these circumstances, it is an inner, a sacred necessity that the essence of reason as distinguished from reason itself be at last identified with it and the divine being thus be apprehended, realised, as the essence of reason. It is on this necessity that the great historical significance of speculative philosophy rests. The proof of the proposition that the divine essence is the essence of reason or intelligence lies in the fact that the determinations or qualities of God, in so far as they are rational or intelligible and not determinations of sensuousness or imagination, are, in fact, qualities of reason.

“God is the infinite being or the being without any limitations whatsoever.” But what cannot be a limit or boundary on God can also not be a limit or boundary on reason. If, for example, God is elevated above all limitations of sensuousness, so, too, is reason. He who cannot conceive of any entity except as sensuous, that is, he whose reason is limited by sensuousness, can only have a God who is limited by sensuousness. Reason, which conceives God as an infinite being, conceives, in point of fact, its own infinity in God. What is divine to reason is also truly rational to it, or in other words, it is a being that perfectly corresponds to and satisfies it. That, however, in which a being finds satisfaction, is nothing but the being in which it encounters itself as its own object. He who finds satisfaction in a philosopher is himself of a philosophical nature. That he is of this nature is precisely what he and others encounter in this satisfaction. Reason “does not, however, pause at the finite, sensuous things; it finds satisfaction in the infinite being alone” – that is to say, the essence of reason is disclosed to us primarily in the infinite being.
...
The necessary being is one that it is necessary to think of, that must be affirmed absolutely and which it is simply impossible to deny or annul, but only to the extent to which it is a thinking being itself. Thus, it is its own necessity and reality which reason demonstrates in the necessary being. — Feuerbach

https://www.marxists.org/reference/archive/feuerbach/works/future/future0.htm

God's non-existence, the Reimann hypothesis, and being happy tomorrow aren't one of the Ys defined in 1 and assumed to exist in 2 (what would it even mean to say that God's non-existence exists?). — Michael

I'm arguing, as per my premise about the sources of uncertainty, that...

2. if Y exists then X knows all properties of Y (a consequence of all Ys being in X's mind) — Isaac

As to...

what would it even mean to say that God's non-existence exists?

...it's a property of the entire world. The world is such that it contains nothing answering the description of god.

You can't go from:

1. If Y exists then X knows that Y exists

to:

2. If Y does not exist then X knows that Y does not exist

Ontological solipsism only entails 1. Your reasoning to 2 is fallacious.

You can't go from:

1. If Y exists then X knows that Y exists

to:

2. If Y does not exist then X knows that Y does not exist — Michael

Seeing as I haven't, I'm not sure why you're mentioning this.

Here's my argument...

1. render all that is the case as the set {Y,Y,Y,Y....} for all Ys
2. if Y exists then X knows all properties of Y (a consequence of all Ys being in X's mind)
3. X therefore cannot ever be wrong about what is the case (since what is the case is entirely constituted of all the Ys)
4. Z (our epistemic solipsist) entertains a possible world in which 2 is true.
5. Z then has to admit that in that possible world X cannot be wrong about what is the case (whether X knows this or not is irrelevant)
6. no X is ever in a situation where they cannot be wrong about what is the case.

5 and 6 are a contradiction. Z has to either reject 5 or reject 6.

Maintaining 6, Z rejects 5.

If Z rejects 5, they cannot also coherently claim scepticism about whether 5 is the case or not. — Isaac

It mentions neither of the propositions you used.

Seeing as I haven't, I'm not sure why you're mentioning this. — Isaac

Ontological solipsism claims:

1. If Y exists then Y is a facet of my mind
2. If Y is a facet of my mind then I know that Y exists
3. Therefore, if Y exists then I know that Y exists

That's it. You, somehow, want to say that this entails:

4. If Y (e.g. God) does not exist then I know that Y does not exist

And even:

5. I know what will happen tomorrow
6. I know what would have happened had I done things differently
7. I know whether or not the Reimann hypothesis is true

But these are all invalid inferences. Ontological solipsism just doesn't entail that I know everything. It only entails that if Y exists then I know that Y exists. There is more to knowledge than just knowledge of what exists.

these are all invalid inferences. — Michael

Just saying it over and over is pointless. You're not my teacher. We're equals here, having a discussion (or supposed to be). I've tried to explain why I think they are valid inferences. I've even referenced that explanation twice now. You've not even acknowledged it, let alone addressed it. If you're just here to lecture me I'm not interested.

Just saying it over and over is pointless. You're not my teacher. We're equals here, having a discussion (or supposed to be). I've tried to explain why I think they are valid inferences. I've even referenced that explanation twice now. You've not even acknowledged it, let alone addressed it. If you're just here to lecture me I'm not interested. — Isaac

We have to use free logic for this (classical logic doesn't allow for "p does not exist"):

1. ∀p: ∃x(x=p) → K(∃x(x=p))
2. ∀p: ¬∃x(x=p) → K(¬∃x(x=p))

There is no rule of inference that lets you derive 2 from 1.

There is no rule of inference that lets you derive 2 from 1. — Michael

We're going round in circles. Nowhere in my argument do I claim, imply, or require deriving 2 from 1.

We're going round in circles. Nowhere in my argument do I claim, imply, or require deriving 2 from 1. — Isaac

You're saying that if ontological solipsism is true then I know that God doesn't exist.

You're saying that if ontological solipsism is true then I know that God doesn't exist. — Michael

No.

Here's my argument...

1. render all that is the case as the set {Y,Y,Y,Y....} for all Ys
2. if Y exists then X knows all properties of Y (a consequence of all Ys being in X's mind)
3. X therefore cannot ever be wrong about what is the case (since what is the case is entirely constituted of all the Ys)
4. Z (our epistemic solipsist) entertains a possible world in which 2 is true.
5. Z then has to admit that in that possible world X cannot be wrong about what is the case (whether X knows this or not is irrelevant)
6. no X is ever in a situation where they cannot be wrong about what is the case.

5 and 6 are a contradiction. Z has to either reject 5 or reject 6.

Maintaining 6, Z rejects 5.

If Z rejects 5, they cannot also coherently claim scepticism about whether 5 is the case or not. — Isaac

If you've no interest in it, that's fine, but it's pointless ignoring it and refuting something else.

You're saying that if ontological solipsism is true then I know that God doesn't exist. — Michael

No. — Isaac

If I don't know that God doesn't exist then I don't know everything.

As I said before, you equivocate on the meaning of "what is the case". Given 1 and 2 it just refers to what exists, and so your conclusion is only that X is not wrong that Y exists. But that he is not wrong that Y exists isn't that he cannot be wrong. As I have repeatedly said, there is more to knowledge than just knowledge of what exists. There is knowledge of what doesn't exist, there is knowledge of what will happen in the future, there is knowledge of what could have happened, there is knowledge of maths. None of this knowledge is accounted for in your argument.

Given 1 and 2 it just refers to what exists — Michael

No, I specifically included the properties of what exists. And I've been through all this. You even asked me what it would mean and I replied that I consider God's nonexistence to be a property of the world (which exists). It is such that there's no thing in it matching the description of god. You ignored my reply completely and are now acting as if I hadn't said anything on the matter.

No, I specifically included the properties of what exists. And I've been through all this. You even asked me what it would mean and I replied that I consider God's nonexistence to be a property of the world (which exists). It is such that there's no thing in it matching the description of god. You ignored my reply completely and are now acting as if I hadn't said anything on the matter. — Isaac

The future is not a property of things that exist in the present. Neither are counterfactuals. Neither is the decimal notation of pi.

It is such that there's no thing in it matching the description of god.

You can't go from "nothing I know of is God" to "I know that God doesn't exist".