Idealism

Idealism holds that reality is fundamentally mental rather than material — what we call "matter" is a structure appearing within, or dependent on, mind, rather than mind being one more thing built out of independently-existing matter. Berkeley's classical slogan is esse est percipi: to exist is to be perceived (in his version, ultimately by God, which keeps the world existing when no finite mind is looking).

Its chief appeal is dissolving rather than solving the mind-body problem: if mind is basic and matter derivative, there is no gap between physical process and subjective experience to explain, because there was never a separate physical substance in the first place. Its standing cost is the opposite direction: explaining why the "external world" behaves with such stubborn, shared, law-governed consistency, and resists an individual perceiver's will so reliably, if it is not a separate stuff at all but only ever perception.