Lecture I

We take hold of a warm object, for example. The scientist will tell us: What you are calling the heat or warmth is the effect on your own nerves. Objectively, there is the movement of molecules and atoms. These you can study, after the laws of mechanics. So then they study the laws of mechanics, of atoms and molecules; indeed, for a long time they imagined that by so doing they would at last contrive to explain all the phenomena of Nature. Today, of course, this hope is rather shaken. But even if we do press forward to the atom with our thinking, even then we shall have to ask—and seek the answer by experiment—How are the forces in the atom? How does the mass reveal itself in its effects,—how does it work? And if you put this question, you must ask again: How will you recognize it? You can only recognize the mass by its effects.