  Although osmosis is a familiar phenomenon, and of pivotal importance in
natural systems, it is seldom explained how it might work on the molecular
level (if treated at all in physics text books). The standard treatment of
osmosis in thermodynamics employs the concept of the chemical potential and
does not give any clues how the process "really" works. On the other hand one
may also encounter conflicting qualitative and molecular explanations of
osmosis. We use the case of osmosis in the present paper to elucidate different
ways to "explain" and "understand" physical phenomena. The role of qualitative
understanding of physical phenomena is emphasized. However, as the case of
osmosis demonstrates, there may be a big gap between the abstract macroscopic
theory and molecular conceptions of the mechanism.
