When several people simultaneously watch a rat in a maze, or any other example of what we should naturally regard as matter in motion, there is by no means complete identity between the physical events which happen at the surface of their eyes and constitute the stimuli to their perceptions. There are differences of perspective, of light and shade, of apparent size, and so on, all of which will be reproduced in photographs taken from the places where the eyes of the several observers are. These differences produce differences in the reactions of the observers--differences which a quite unthinking person may overlook, but which are familiar to every artist. Now it is contrary to all scientific canons to suppose that the object perceived, in addition to affecting us in the way of stimulus and reaction, also affects us directly by some mystical epiphany; certainly it is not what any behaviourist would care to assert. Our knowledge of the physical world, therefore, must be contained in our reaction to the stimulus which reaches us across the intervening medium; and it seems hardly possible that our reaction should have a more intimate relation to the object than the stimulus has. Since the stimulus differs for different observers, the reaction also differs; consequently, in all our perceptions of physical processes there is an element of subjectivity. If, therefore, physics is true in its broad outlines (as the above argument supposes), what we call “perceiving” a physical process is something private and subjective, at least in part, and is yet the only possible starting-point for our knowledge of the physical world.