Mind is to the nervous system as rotation is to the wheel. Any mechanical motion results in some kind of inner experience. All particles possess a bit of inner life, even atoms are conscious. Reductive Materialism resembles animists, but consciousness is purely mechanical property that we lack the tools to measure. Mind is the fundamental substance of the world.
Experience is mental in character.
The existence of inner experience is undeniable, but to an idealist the existence of an external world is not certain. World looks like a dream state. Idealism suggests - developing a wholly mental science based on manipulation and observation of states of consciousness, rather than states of matter. Posits the existence of a single substance possessing both mental and physical attributes. The body is seen as inferior to the soul, even down right evil.