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Are there any philosophical arguments to disprove or weaken solipsism?
What justification(s) exist for the generally-agreed-upon axiom that a physical world beyond our mind both exists, and is knowable? This seems to be taken for granted, but rarely wrestled with in my experience. If no justification exists, does it not lead to inevitable epistemological agnosticism, removing any basis from which a man may claim to have any knowledge at all?
Justifications? Well, yes:
You experience a world, right. It seems to be full of other people, like you. They seem to also experience a world, and by comparing experiences and building machines which measure the world, it seems that our experiences of the world are the same. We can also see how other people stop and start existing without the world doing so.
That leaves us three options.
Option 1: There is a physical, objective world beyond the mind. You are like one of the other people, a part of the world. The world is not a part of you. The world didn't start with you and will not end with you.
Option 2: There is a physical, objective world beyond the mind, but it's not the one you are experiencing. You are in fact being induced to hallucinate this world instead of the real one. Probably because aliens somehow want to use your body as an energy source, even though that claim makes so scientific sense and is about the worst plot device ever. Nothing you do can break this illusion, there is no blue or red pill.
Option 3: There is no physical world outside your mind, there is only you. You are alone and god. For some reason, when you started existing, you for no particular reason started hallucinating that you were human being that was a couple of years old. And for some really inexplicable reason, you hallucinate a reality over which you have no control. Although you can make yourself a rock star with infinite amounts of money and sex, you hallucinate yourself sitting in front of a computer asking interesting philosophical questions on a website. Nothing you do seem to be able to break the hallucination that the world you hallucinate is not a hallucination.
Now, option 2 is clearly absurd, and option 3 doesn't make any sense. If the world really was your hallucination, why aren't you hallucinating something better?
So yes, there is justifications for assuming that there is an objective world outside of your mind: Everything you experience tells you there is, and nothing you do can break that experience. The only reasonable assumption in that case is that the world in fact is objective and exists outside and independently of your mind.
"What justification(s) exist for the generally-agreed-upon axiom that a physical world beyond our mind
bothexists,and is knowable?" (I don't know what you mean by "knowable".)
But you might like to see: What is an 'argumentum ad lapidem'?
In an essay titled "Berkeley's Crossroads," Borges formulates the problem with realism:
By general rule, substance is assigned only size, while all other characteristics – color, taste, and sound – are considered beholden to a borderland between the spiritual and the material, an intermediate universe or the outskirts that are forged, in a continual and secret collaboration, between spatial reality and our organs of perception. This conjecture suffers from grave flaws. The pure and simple naked expanse that, according to the dualist and materialists [realist], composes the essence of the world is a useless trifle: blind, vain, formless, without bulk, neither soft nor hard, an abstraction that no one manages to visualize. The act of assigning it substance is a desperate measure of anti-metaphysical prejudice that completely fails to deny the essential reality of the external world and that takes refuge in a shady compromise by tossing it a verbal bit of charity: a hypocrisy comparable to the concept of atoms, imagined solely as a defense against the idea of infinite divisibility.
Borges dismissal of realism isn't particularly interesting because this is a classic application for the identity of indiscernibles. Until a distinction between a real world and an imagined world can be tested, it doesn't matter which one it is...it's like the question: "are other human beings conscious?"
