Ok.. but do you agree that something that is real (a brain in a jar for example) is experiencing these things (rock, paint, etc.) ? I'm the first to agree that objective reality cannot be observed. All we perceive of it is our own subjective reality. But fact remains that these 'impressions' must be caused by something, something that is objective. I agree that it cannot be proven to exist. But it can be shown to exist beyond a reasonable doubt.