Proof Mind is Fundamental and Matter Doesn't Exist

[Angrboda]

Argument:
1. a metaphysically solipsist world (a world where only a mind exists) cannot be proven false due to epistemic limitations.
2. it is unreasonable to presume solipsism is impossible given 1, therefore it must be reasonably granted solipsism is possible.
3. given 2, it is possible for mind to exist in a solipsist (immaterial) world while it is not for matter.
4. there is therefore something that it true of mind but not of matter. this means they cannot be the same thing and mind is not reducible to matter.
5. substance dualism has been proven false due to the interaction problem (substances can only interact via shared properties and substances cannot be fundamental and share properties).
6. therefore, all is mind and monistic idealism entails.

It doesn't follow that because the existence of a solipsistic world can't be proven false that it is therefore possible. There can be reasons why it can't be proven false in spite of it actually being a false proposition. Just as in math, that there are propositions which can't be proven to be true doesn't imply that they are false. Inability to disprove solipsism doesn't imply that it is possible. The two properties, provability and possibility, are not directly related. Just as material mind must be directly evidenced, so solipsism requires direct argument for its possibility.

Moreover, you're assuming that a full explanation of mind won't show how matter is essential to mind. That's an argument from ignorance.