In the course of the last three chapters we have traced the development of the idea of perception from the time of the Greeks to the present day. In so doing we have been obliged to omit reference to a great many more theories than we have been able to mention, and at the same time to simplify the latter considerably, though it is hoped without distortion. During the present century alone, a great many philosophical theories have been advanced to explain the “fact” of perception, and much scientific investigation has been devoted to the physical “facts” of the process. And there still remains much to be done. What some readers may be inclined to feel, as they begin the present chapter, is that the “fact” of perception is something which we shall come to understand better only as a result of the discovery of more “facts” in the scientific sense. Why philosophise, they will say, when the only profitable source of knowledge is scientific enquiry? Will not science, as it elucidates the facts, provide its own commentary upon them? What is the use of generalising upon inadequate evidence?