The fact that it is possible for a number of people to perceive the same noise or the same coloured pattern obviously depends upon the fact that a physical process can travel outward from a centre and retain certain of its characteristics unchanged, or very little changed. The most notable of such characteristics is frequency in a wave-motion. That, no doubt, affords a biological reason for the fact that our most delicate senses, sight and hearing, are sensitive to frequencies, which determine colour in what we see and pitch in what we hear. If there were not, in the physical world, processes spreading out from centres and retaining certain characters practically unchanged, it would be impossible for different percipients to perceive the same object from different points of view, and we should not have been able to discover that we all live in a common world. We come now to the process in the percipient’s body, in so far as this can be perceived by an outside observer. This raises no new philosophical problems, because we are still concerned, as before, with the perception of events outside the observer’s body. The observer, now, is supposed to be a physiologist, observing, say, what goes on in the eye when light falls upon it. His means of knowing are, in principle, exactly the same as in the observation of dead matter. An event in an eye upon which light is falling causes light-waves to travel in a certain manner until they reach the eye of the physiologist. They there cause a process in the physiologist’s eye and optic nerve and brain, which ends in what he calls “seeing what happens in the eye he is observing”. But this event, which happens in the physiologist, is not what happened in the eye he was observing; it is only connected with this by a complicated causal chain. Thus our knowledge of physiology is no more direct or intimate than our knowledge of processes in dead matter; we do not know any more about our eyes than about the trees and fields and clouds that we see by means of them. The event which happens when a physiologist observes an eye is an event in him, not on the eye that he is observing. We come now at last to the question of self-observation, which we have hitherto avoided. I say “self-observation” rather than “introspection”, because the latter word has controversial associations that I wish to avoid. I mean by “self-observation” anything that a man can perceive about himself but that others, however situated, cannot perceive about him. What follows is only preliminary, since the subject will be discussed at length in Chapter XVI.