"Version: 20250901"--Title page verso.Includes bibliographical references.The drunkard and the policeman -- part I. Theoretical background. 1. The ISM and star formation -- 1.1. The ISM as a Flowing continuum -- 1.2. the components of the ISM2. The hydrodynamic equations -- 2.1. The fluid approach -- 2.2. The conservation equations -- 2.3. Constitutive relations -- 2.4. The fluid equations from…