This section analyzes the development from classical physics to relativity and quantum theory, explaining how traditional concepts of matter and space have been transformed. Russell interprets key ideas such as electrons, quanta, relativity, and tensor methods in a philosophical way, showing that physics has become increasingly abstract. He emphasizes that physical concepts represent mathematical structures rather than simple descriptions of material substance. Through discussions of measurement and scientific laws, he explores how knowledge in physics is constructed. As a result, matter is no longer understood as a concrete entity but as part of an abstract theoretical framework.