Popular metaphysics divides the known world into mind and matter, and a human being into soul and body. Some--the materialists--have said that matter alone is real and mind is an illusion. Many--the idealists in the technical sense, or mentalists, as Dr. Broad more appropriately calls them--have taken the opposite view, that mind alone is real and matter is an illusion. The view which I have suggested is that both mind and matter are structures composed of a more primitive stuff which is neither mental nor material. This view, called “neutral monism”, is suggested in Mach’s _Analysis of Sensations_, developed in William James’s _Essays in Radical Empiricism_, and advocated by John Dewey, as well as by Professor R. B. Parry and other American realists. The use of the word “neutral” in this way is due to Dr. H. M. Sheffer,[15] formerly of Harvard, who is one of the ablest logicians of our time. [15] See Holt’s _Concept of Consciousness_, preface. Since man is the instrument of his own knowledge, it is necessary to study him as an instrument before we can appraise the value of what our senses seem to tell us concerning the world. In Part I we studied man, within the framework of common-sense beliefs, just as we might study clocks or thermometers, as an instrument sensitive to certain features of the environment, since sensitiveness to the environment is obviously an indispensable condition for knowledge about it. In Part II we advanced to the study of the physical world. We found that matter, in modern science, has lost its solidity and substantiality; it has become a mere ghost haunting the scenes of its former splendours. In pursuit of something that could be treated as substantial, physicists analysed ordinary matter into molecules, molecules into atoms, atoms into electrons and protons. There, for a few years, analysis found a resting-place. But now electrons and protons themselves are dissolved into systems of radiations by Heisenberg, and into systems of waves by Schrödinger--the two theories amount mathematically to much the same thing. And these are not wild metaphysical speculations; they are sober mathematical calculations, accepted by the great majority of experts.