Published online by Cambridge University Press: 14 March 2022
A physical theory intends to provide us with a picture of a certain domain of physical phenomena. We have become accustomed to deal rather with physical pictures than with phenomena themselves. This is indispensable, even if the number of phenomena contained in the picture is so large that we cannot have them in mind all at the same time or if the phenomena are not directly accessible to our senses and cannot be studied without the help of a set of very complicated apparatus.
1 It is a great pleasure to the author to express his very sincere thanks to Professor E. Gaviola for his kind interest in this work and for his, indeed, very helpful criticism.