Uh, states of mind are easily proven and we have a whole slew of medication for depression and any other mental problem you can possibly have.
This is going to turn into philosophy of science so its gonna take a lot of fucking walls of text...Ill try and keep it as short as possible
We never know states of mind directly, only undirectly through measurements of brain activity, which only works as a MODEL, science doesnt have today the pretention of knowledge, just funcionality.
Following that, the fact that it is, sometimes, funcional doesnt render it true, just an aproximation, and that is only under the assumption that there is something we can actually aproximate too (this is a Lakatos example not mine), its like walking in the absolute dark, how do you know you are getting closer to anywhere? You can be sure through experience (not really, but for simplifications) that certain medicine appears to help patients, do you know how many errors are made for actual results? The scientific method does not actually exist, and it is because of "unscientific science" that people thought that maybe the planet was not the center of the universe, going against the grain.
[citation needed]
Kuhn, Lakatos, Feyerabend