Welcome, @WhoKnows, to our dance! 
I think the whole idea of solipsism is that one mind is all there is. That the Universe comprises only that one thing…?
It doesn’t need to. It only needs to be “the only thing that exists”, I think. 
Thanks for the welcome @ PatternChaser
There are varying types of Solipsism.
All appear to be centered on one’s own “sureness of existence” and I think the problem with it is that it doesn’t account for anyone’s else’s mindfulness and thus is not even human centric but only self-centric.
I can understand the idea of Solipsism perhaps being true ONLY from the pov of a Source Mind, whereby that mind creates formations which it can inhabit in order to produce the illusion of others, and this universe being one of perhaps multitude universes.
This would signify that we humans are all part of this overall mindfield but not any one of us being the central mind that creates the illusion of “others”…
It’s impossible to take ALL logical possibilities seriously. People entertain possibilities that pique their interest, which is fine - but the motivation is not just “it’s logically possible”. Solipsism is like that.
It’s “impossible”? Well, I try my best. 
Although to consider “all” possibilities might be beyond any human! 
But within my finititude (?!), I do try my best. That which is possible, and has not (yet?) been refuted or falsified, remains possible, and therefore available for serious consideration, if we have a mind to…
It needs to be the only thing that exists for the claim to be true. But the truth conditions are unclear when the words are used across two different realities (supposedly with different truth conditions).
We can’t meaningfully say that the imaginary dog Goofy is the only thing that exists. Whether it’s possible in a fictional reality is beside the point when we’re using words whose meanings refer to a factual reality.
Likewise, we can’t meaningfully say that a mind is the only thing that exists merely because it’s logically possible in a different reality. The words are used under different truth conditions.
I mean not necessarily, I think. You could have a brain in an empty void of spacetime and the reality we perceive just be a dreamlike state.
Sure, in a fictional reality where a brain is not a biological organ etc.
Yes. isn’t this similar to Kastrup’s idealism with one great non-metacognative mind of which we are all in his words, dissociated alters?
True that. I didn’t know of anyone else working with this idea. A quick ai overview shows me Kastrup’s idealism is much the way I have learned to think about things, mainly through my own experience since my late teens…
I haven’t encountered anything which seriously challenges the concept.
One thing I will add is that - yes - while the blank slate experience is what causes the disassociation, it needn’t be permanent. There are degrees of reconnection and recognition…
I enjoy Kastrup’s model although I don’t think it can be demonstrated to be true.
When you think about it, Vedantic traditions and even some forms of Christian Neoplatonism emphasise our underlying unity with God. “We are all one” that kind of thing. You can see how this naturally leads to the idea that our apparent separateness is ultimately an illusion and that, at some deeper level, we are manifestations of the same underlying reality.
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Yes, it does. But I commented only that the “alleged mind” need not “think that it’s the only thing that exists”, as long as it IS the only thing that exists.
I’d say its existence is unknowable and therefore irrelevant. All we have is the claim, and it consists of words whose meanings are constrained by their causal history and ordinary use. But the claim denies the ordinary uses of the words ‘mind’ and ‘exist’.
A “mind” without a cause supposedly “exists” without sufficient reason, but that’s arguably not what the words ‘mind’ and ‘exist’ mean.
There are (very) many things in our lives that are “unknowable”, but we seem to manage. Many of them do NOT seem “irrelevant” to me… 
[quote=“Tom_Storm, post:51, topic:1865”]
I enjoy Kastrup’s model although I don’t think it can be demonstrated to be true.
Nothing in particular, I agree. But…perhaps when we look at many things, these point to that most likely being the case…
When you think about it, Vedantic traditions and even some forms of Christian Neoplatonism emphasise our underlying unity with God. “We are all one” that kind of thing.
Yes. Given our individuate status, such concepts are difficult to integrate into personal perspective. “We are all one” becomes an acknowledgment for those who come to that realisation through whatever means they do so. Perhaps still thin on the ground, but active nonetheless…
You can see how this naturally leads to the idea that our apparent separateness is ultimately an illusion and that, at some deeper level, we are manifestations of the same underlying reality.
And - to be fair - “some deeper level” is a terrifying possibility for all of us who naturally begin at the Blank Slate default and the “struggle” to remain wilfully ignorant about such concepts, is real for many - and given the usual gate-kept alternatives on offer, who can blame them?