the mirror of the universe. All philosophers, then, agree on this point. Only if when we consider any other given place in the universe we can regard the action of all matter as passing through it without resistance and without loss, and the photograph of the whole as translucent: here there is wanting behind the plate the black screen on which the image could be shown. Our 'zones of indetermination' play in some sort the part of the screen. They add nothing to what is there; they effect merely this: that the real action passes through, the virtual action remains.
This is no hypothesis. We content ourselves with formulating data with which no theory of perception can dispense. For no philosopher can begin the study of external perception without assuming the possibility at least of a material world, that is to say, in the main, the virtual perception of all things. From this merely possible material mass he will then isolate the particular object which I call my body, and, in this body, centres of perception: he will show me the disturbance coming from a certain point in space, propagating itself along the nerves and reaching the centres. But here I am confronted by a transformation scene from fairyland. The material world which surrounds the body, the body which shelters the brain, the brain in which we distinguish centres, he abruptly dismisses; and, as by a magician's wand, he conjures up, as a thing
