The above discussion does not pretend to cover the whole of the ground that has to be covered in discussing the definition of “knowledge”. There are other points of view, which are also necessary to a complete consideration of the question. But these must wait until, after considering the physical world, we come to the discussion of man as viewed from within. PART II THE PHYSICAL WORLD CHAPTER IX THE STRUCTURE OF THE ATOM In all that we have said hitherto on the subject of man from without, we have taken a common-sense view of the material world. We have not asked ourselves: what _is_ matter? Is there such a thing, or is the outside world composed of stuff of a different kind? And what light does a correct theory of the physical world throw upon the process of perception? These are questions which we must attempt to answer in the following chapters. And in doing so the science upon which we must depend is physics. Modern physics, however, is very abstract, and by no means easy to explain in simple language. I shall do my best, but the reader must not blame me too severely if, here and there, he finds some slight difficulty or obscurity. The physical world, both through the theory of relativity and through the most recent doctrines as to the structure of the atom, has become very different from the world of everyday life, and also from that of scientific materialism of the eighteenth-century variety. No philosophy can ignore the revolutionary changes in our physical ideas that the men of science have found necessary; indeed it may be said that all traditional philosophies have to be discarded, and we have to start afresh with as little respect as possible for the systems of the past. Our age has penetrated more deeply into the nature of things than any earlier age, and it would be a false modesty to over-estimate what can still be learned from the metaphysicians of the seventeenth, eighteenth and nineteenth centuries.