we all start from "naive realism," i.e., the doctrine that things are what they seem. we think that grass is green, that stones are hard, and that snow is cold. but physics assures us that the greenness of grass, the hardness of stones, and the coldness of snow, are not the greenness, hardness, and coldness that we know in our own experience, but something very different. the observer, when he seems to himself to be observing a stone, is really, if physics is to be believed, observing the effects of the stone upon himself.