A unified approach leading to the understanding of the liquid state has been developed. The liquid state is characterized by the onset of non-linear anharmonic vibrations at T-g, and is connected with the first appearance of vacancies. The second boundary temperature for the liquid state "T-c" is understood as the temperature at which (as a result of an overabundance of holes) the continuous phase can no longer exist and the "foamy" structure of the liquid disintegrates into clusters. The p(c) value is connected with the cohesive energy density and also depends on Fitter’s acentric factor which itself can characterize the type of bonding (local versus dispersed) of the molecule within the liquid phase. The critical volume of individual compounds seems to be mainly connected with the high-frequency electronic vibrations of individual atoms and their polarizabilities. The critical volume is usually about 50-55 times larger than the summation of individual electronic polarizabilities of the atoms from which the characteristic substance is made.