A treatment of liquid helium is given that is a simplified version of that in a former paper. The assembly, consisting of vapour and liquid, is treated as an equilibrium mixture of aggregates of all sizes, an aggregate being defined as a number of atoms which together form a bound quantum-mechanical state. By making plausible assumptions about the energy levels of these aggregates a model is found which shows both liquefaction and a generalized Bose condensation. The model seems to be a satisfactory compromise between the points of view of Landau and of London and Tisza, and leads to a synthesis of their two explanations of the appearance of superfluidity. It also provides a physical basis for the model assumed by F. London in 1939.