 I have settled down to the task of writing these lectures and have drawn up my chairs to my two tables. Two tables? Yes, there are duplicates of every object about me. Two tables. Two chairs. Two pens. This is not a very profound beginning to a course which ought to read transcendent levels of scientific philosophy. But we cannot touch bedrock immediately. We must scratch a bit at the surface of things first. And whenever I begin to scratch, the first thing I strike is my two tables. One of them has been familiar to me from earliest years. It is a commonplace object of that environment which I call the world. How shall I describe it? It has extension. It is comparatively permanent. It is colored. Above all, it is substantial. By substantial, I do not merely mean that it does not collapse when I lean upon it. I mean that it is constituted of substance. And by that word, I am trying to convey to you some conception of its intrinsic nature. It is a thing, not like space, which is a mere negation, nor like time, which is heaven knows what. But that will not help you to my meaning because it is the distinctive characteristic of a thing to have this substantiality. And I do not think substantiality can be described better than by saying that it is an ordinary table. And so we go round in circles. After all, if you are a plain common sense man, not too much worried with science, you will be confident that you understand the nature of an ordinary table. Table number two is my scientific table. It is a more recent acquaintance, and I do not feel so familiar with it. It does not belong to the world previously mentioned, that world which spontaneously appears around me when I open my eyes. It is part of a world which has forced itself on my attention. My scientific table is mostly emptiness. Sparsely scattered in that emptiness are numerous electrical charges rushing about with great speed. But their combined bulk amounts to less than a billionth of the bulk of the table itself. Notwithstanding its strange construction, it turns out to be an entirely efficient table. It supports my writing paper as satisfactorily as table number one. For when I lay the paper on it, the little electrical particles with their headlong speed keep on hitting the underside, so that the paper is maintained in a ping-pong-like fashion at a nearly steady level. If I lean upon this table, I shall not go through. Or to be strictly accurate, the chance of my elbow going through my scientific table is so excessively small that it can be neglected. Reviewing their properties one by one, there seems to be nothing to choose between the two tables for ordinary purposes. But when abnormal circumstances come about, then my scientific table shows itself superior. If the house catches fire, my scientific table will dissolve quite naturally into scientific smoke, whereas my familiar table undergoes a metamorphosis of substance which I can only regard as miraculous. There is nothing substantial about my second table. It is nearly all empty space, space pervaded, it is true, by fields of force. But these are assigned to the category of influences, not of things. Even in the minute part, which is not empty, we must not assume there is substance. In dissecting matter into electrical charges, we have traveled far from the picture which first gave rise to the conception of substance. The whole trend of modern scientific views is to break down the separate categories of things, influences, forms, etc., and to substitute a common background of all experience. Whether we are studying a material object, a magnetic field, a geometrical figure, or a duration of time, our scientific information is summed up in measures. Neither the apparatus of measurement nor the mode of using it suggests that there is anything essentially different in these problems. The measures themselves afford no ground for a classification by categories. We feel it necessary to concede some background to the measures, an external world. But the attributes of this world, except insofar as they are reflected in the measures, are outside scientific scrutiny. Science has revolted against attaching the exact knowledge contained in these measurements to a traditional picture gallery of conceptions which convey no information and obtrude irrelevancies into the scheme of knowledge. I will not here stress further the non-substantiality of electrons, since it is scarcely necessary to the present line of thought. Conceive them as you will. There is a vast difference between my scientific table, with its substance thinly scattered in specks in a region mostly empty, and the table of everyday life which we regard as solid reality. It makes all the difference in the world whether the paper before me is poised on a swarm of flies and sustained by a series of tiny blows from the swarm underneath, or whether it is supported because there is substance below it. All the difference in conception at least, but no difference to my practical task in the lectures. Modern physics has by delicate test and remorseless logic assured me that my second scientific table is the only one which is really there, wherever there may be. On the other hand, I need not tell you that modern physics will never succeed in exercising that first table, strange compound of external nature, mental imagery, and inherited prejudice, which lies visible to my eyes and tangible to my grasp. However, we must bid goodbye to it for the present, for we are about to turn from the familiar world to the scientific world revealed by physics. To speak paradoxically of two worlds, are they not really two aspects or two interpretations of one in the same world? Yes, no doubt they are ultimately to be identified after some fashion, but the process by which the external world of physics is transformed into a world of familiar is outside the scope of physics. And so the world studied according to the methods of physics remain detached from the world familiar to consciousness. Provisionally, we regard the table, which is the subject of physical research, as altogether separate from the familiar table. It is true that the whole scientific inquiry starts from the familiar world, and in the end it must return to the familiar world. But the part of the journey over which the physicist has charged is in foreign territory. Until recently there was a much closer linkage. The physicist used to borrow the raw materials of his world from the familiar world, but he does so no longer. His raw materials are electrons, quanta, potentials, Hamiltonian functions, etc., and he is nowadays careful to guard these from contamination by conceptions from the common world. There is a familiar table parallel to the scientific table, but there is no familiar electron or quantum parallel to these scientific entities. We do not even desire to manufacture a familiar counterpart to these things. After the physicist has quite finished his world building a linkage or identification is allowed, but premature attempts at linkage have been found to be entirely mischievous. Science aims at constructing a world which is a symbolic counterpart of the world of commonplace experience. It is not necessary that every individual symbol should represent something in common experience. The man in the street making this demand for concrete explanations of the things referred to in science will be disappointed. It is like our experience in learning to read. That which is written in a book is symbolic of a story in real life. The whole intention of the book is that ultimately a reader will identify some symbol, say bread, with one of the conceptions of familiar life. But it is mischievous to attempt such identifications prematurely before the letters are strung into words and the words into sentences. The symbol E is not the counterpart of anything in familiar life. Letters are abstract and sooner or later one has to realize it. The external world of physics has thus become a world of shadows. In removing our illusions we have removed the substance of the world. Later perhaps we may inquire whether in our zeal to cut out all that is unreal we may not have used the knife too ruthlessly. Perhaps reality is a child which cannot survive without its nurse's illusion. But if so that is of little concern to the scientist who has good reasons for pursuing his investigations in the world of shadows and is content to leave to the philosopher the determination of its status in regard to reality. In the world of physics we watch a shadow-graphed performance of the drama of familiar life. The shadow of my elbow rests on the shadow table as the shadow ink flows over the shadow paper. It is all symbolic and as a symbol the physicist leaves it. Then comes the alchemist Mind who transmutes the symbols. The sparsely spread nuclei of electric force become a tangible solid. Their restless agitation becomes the warmth of summer, a gorgeous rainbow. In the transmuted world new significances arise which are scarcely to be traced in the world of symbols so that it becomes a world of beauty and purpose and the last suffering and evil. The frank realization that physical science is concerned with a world of shadows is one of the most significant of recent advances. I do not mean that physicists are to any extent preoccupied with the philosophical implications of this. From their point of view it is not so much a withdrawal as an assertion of freedom. But I am convinced that a just appreciation of the physical world as it is understood by science today carries with it a feeling of open-mindedness towards a wider significance. The path of science must be pursued for its own sake, irrespective of the views it may afford of a wider landscape. In this spirit we must follow the path whether it leads to the hill of vision or the tunnel of obscurity. Thanks for watching.