How does a dream differ from waking life? In a fairly coherent dream everything seems quite real, and it is rare, I think, with most people to know in a dream that they are dreaming. Now, if a dream is sufficiently vivid and coherent, all physical inferences are just as valid in it as they are in waking life. In a hazy or imperfect dream, it is true, things melt into one another unexpectedly and unaccountably; we fly, remove mountains, and stop runaway horses with a finger. But there is nothing in the mere nature of a dream to hinder it from being an exact copy of waking experience. If I find a stone heavy in one part of my dream, and infer that it is heavy at some subsequent part, the inference will be verified if the dream is coherent enough; I shall go to the stone, lift it up, and find it as heavy as before. And the same thing is true of all inferences of phenomena from phenomena. For physical purposes a dream is just as good as real life; the only difference is in vividness and coherence. What, then, hinders us from saying that life is all a dream? If the phenomena we dream of are just as good and real phenomena as those we see and feel when we are awake, what right have we to say that the material universe has any more existence apart from our minds than the things we see and feel in our dreams? The answer which Berkeley gave to that question was, No right at all. The physical universe which I see and feel, and infer, is just my dream and nothing else; that which you see is your dream; only it so happens that all our dreams agree in many respects. This doctrine of Berkeley's has now been so far confirmed by the physiology of the senses, that it is no longer a metaphysical speculation, but a scientifically established fact.