This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. The results of physical research which are of importance for an understanding of Nature have been compiled in a short and, in so far as the author was capable, a popular presentation. The book concerns bodies as they appear to the authors and as they perceive them directly by means of their eyes. It discusses the whole of the authors' knowledge concerning the atomic structure of matter. The work of the last few decades has allowed them to penetrate still further into the inner structure of matter. The book examines the structure of the atom itself. Finally there exists a whole series of phenomena, which have only been explained on the assumption that, in addition to ponderable matters, there is still something else which fills all interstellar space, a medium which is called the aether.