I n the last chapter I touched incidentally on the sensible motion of sensa within their own fields. Both in it and in the chapter before I talked of the motion of our bodies, and said that the concept of physical Space is based on such motions, interpreted spatially by analogy with our visual fields. I propose now to go considerably more into detail about these matters ; to consider exactly how the concepts of physical Space and Motion are connected, on the one hand with our bodily movements, and on the other with the positions and movements of our sensa in their fields ; and finally to work up to the concept of physical Space-Time. We shall find that the consideration of our own bodies and of the bodies of other observers who can communi­ cate with us about their experiences fills a gap in our concept of physical objects, and is an essential factor in the development of the concept of physical Space.