In this paper, we would like to discuss the problem of the modelisation of the structure and dynamical behaviour of liquids, more generally of dense fluid phases composed of and/or containing in solution, non-rigid molecules. More specifically, we shall focus on the problem of the interpretation of NMR data in anisotropic media, which is of direct relevance in this field of research, keeping in mind that the concepts and ideas presented here may be used for the interpretation of other kinds of (spectroscopic) data. In such a broad and difficult subject, there are many researchers and different schools of thought. We shall try here to present two different models that are used in this field, and which are based on two rather opposite pictures of a liquid, the solid-like picture and the gas-like picture. Emphasis will be particularly put on the former one, giving reasons why we believe it is (much) better than the other. This may upset several people who are supporters of the latter. For this we apologize, but we prefer the reader to draw his own conclusions.