The Silk mass and the Silk–Rees–Ostriker fragmentation condition set basic elements of the physics of the formation of galaxies in the hot expanding Universe. The analyses by Joseph Silk and his group show how galaxy formation is related to the angular fluctuations in the temperature of the thermal cosmic background radiation, the intergalactic clouds of atomic hydrogen, and the plasma in rich clusters of galaxies. Stars are essential parts of galaxies; Joseph has been the leading advocate of the incorporation of empirical knowledge of star formation into cosmogony. These systematic studies by Joseph have played an important part in bringing us to our present tightly constrained picture of how galaxies formed.