Solipsism, as a philosophical doctrine, is usually presented in one of the following ways:
As a metaphysical or ontological thesis where you and only you exist (that you and you alone exist) and other things you experience are the unwitting output or product of your brain or mind. E.g. - you live in a dream of your own making even though you don't necessarily know it and all things you experience are just a hallucination.
As an epistemological challenge from Pyrrhonian-style (philosophical) skepticism. E.g. - How can you disprove the thesis above?
Rarely held, even more rarely defended, many philosophers have taken Solipsism to be the most extreme form of Idealism and startling jarring if it were true. The view has often been treated as a kind of philosophical bogey-man representing a highly undesirable reality.
Many philosophers (Wittgenstein, Descartes, Russell, and others) devote small passages to refuting various strands of Idealism and/or Skepticism. Both Wittgenstein and Russell conceived fairly celebrated and fairly well-known defenses against the advocate of Solipsism.
For Russell, the best explanation for the continuity of experience is that there is a mind-independent reality. Things continue when we don't immediately experience them. When my cat disappears behind a couch, it reappears on the other side after it runs behind implying my direct perception is not required for the continuity of that event. By parsimony then, Russell concluded, Solipsism loses out to Realism.
For Wittgenstein, the problem with Solipsism is that it's mutually incompatible with the nature of language which is required to even formulate the thesis of Solipsism in the first place. Language is fundamentally a public activity (the word 'public' here loses some of its intended meaning in translation - it does mean spoken "in public" rather than that the meanings of words are fixed by communities of people - you learn words from other people). So, the Solipsist gets the order of precedence wrong - they learn difficult advanced concepts that are publicly defined by communities of philosophers and then go about using that vocabulary in the attempt to argue against the Realist. Philosophy, on Wittgenstein's view, uses language to articulate ideas - the language precedes philosophy (although philosophy can make and shape jargon).
Strictly speaking, neither of the two arguments against Solipsism entails that it's not true. I'd like to spend this blog post dismantling Solipsism and offering my own criticism.
'Solipsism' as used in medicine and psychology shares the same etymology as the word used in philosophy. 'Solipsism' as used in medicine describes a symptom of Schizophrenia where people are unable to distinguish differences between their own internal mental life, the external world, other people, and self.
For instance: https://www.tandfonline.com/doi/full/10.1080/09515089.2017.1410877
'Solipsism' in the medical sense is not a philosophical belief or position. It's a symptom that produces great suffering and misery. And it can be measured with a CT scan.
The two uses slightly overlap as we can see. I am focused primarily on the philosophical use (and may elaborate on how the defense here might be useful to the first use - helping people to reestablish the default sense of reality most people have).
So, I take Realism here to minimally be the thesis that:
There are two fundamental and non-overlapping parts that combine to form Reality: the mental and the non-mental. (We'll leave the rest of that story for other metaphysical views to articulate - e.g. mind-body dualism, materialism, etc.)
The mental part lacks total control over the non-mental part. The absence of the mental does not entail the absence of the non-mental. Events occur in the non-mental part that are outside the control of the mental part.
I think that's an essential structure or skeleton of Realist theories (the idea architecture if you will, exhibits the above edifice despite the specific gloss that puts on it). This is a bit vague but should suffice. There's not much hand-waving going there. If pushed to provide a better defense, I could couch the above in more concise, well-understood, and familiar terminology like dependence, supervenience, etc.
For example, another slightly weaker formulation would be something like:
3. REALITY df= A ∪ B
4. A ∩ B = ∅
5a. B CHANGES A
5b. SOME b ⊂ B CHANGES some a ⊂ A such that a does not CHANGE b
The above argument makes few ontological assumptions about other minds, consciousness, and tries to restrict the ontological vocabulary to a stringent few (for parsimony's sake and since I intend it to reveal the skeleton or edifice behind Realist theories).
However, the argument above mirrors several traditional arguments against Solipsism (See Dharmakīrti, Berkeley, Avramides, and Henkel: https://core.ac.uk/download/pdf/145691441.pdf). It differs by focusing on the structure of the argument narrative (as it were) - abstracting the specific distinctions between actions, streams of consciousness, or kinds of ideas seen in the prior ones. I think the other criticism are quite correct and hope to capture the general kind of attack against Skepticism in meta-argument-form.
If granted the above sketch about the edifice of Realist theories, I think Solipsism would immediately collapse into a variant of Realism.
On my view, the prima facie, fundamental, distinction between Solipsism and Realism, is not Idealism but the claim that there is no apparent distinction between some external and internal reality.
Even if Solipsism were true, events occur that are outside the control of your immediate perceptual experience (or mine for that matter haha). You cannot will yourself to fly by merely flapping your "wings" (when one flaps their human arms wildly and comically), even in a Solipsist reality that's similar enough to our own (e.g. - the same physical laws but only conjured by your unconscious mind rather than some external reality).
So, on my view, as long as that distinction is retained (e.g. - between one's immediate perceptual experience and something else) the crux, core, essential part of Realism obtains - that there are two fundamental parts to Reality. A part with limited insight and ability to modify the other. A mostly passive component and one that actively impinges upon elements of the passive (component). The inner mental life and some element external to that largely outside of its control.
Let me try to illustrate that a little more imaginatively: if Solipsism were true, then your mind or brain would be solely responsible for populating your perception of reality with other people (figments though they might only be) - people with distinct personalities, agendas, etc. You have little control over how they act, behave, etc. (despite being only figments of your imagination, on such a view). So, there's little practical difference (little difference period) between Realism and Solipsism.
Therefore, Solipsism collapses into a form of Realism. It serves less as a worrisome instrument of Skepticism and more as a paradigm example about dissolving pseudo-problems in philosophy following deeper analysis.
It's also a different kind of move - placing emphasis on satisfaction conditions rather than on direct proof (although we could parse it as such: p1 X is P if Y p2 Y, Therefore, X is P).
Apparently, the conclusion of the argument above has some precedent:
Others maintain that there are clear distinctions between Realism and Solipsism (implying that my thesis above is fairly original):
The general move that I make above might be worth considering in its own right (e.g. - the move from argument to meta-argument). For example, Crispin Wright also does this in analyzing equivalence classes of skeptical arguments (which he dubs a "Unified Strategy").
The move to a meta-argument from local argument is akin to the Skeptic's move to knock out a full region of knowledge from a local one.