  A merit of this latter approach is that interactions between molecules can be taken into account. Indeed, the approach is not restricted to gases, but also applicable to liquids or solids. The price to be paid, however, is that the probabilities themselves become more abstract. Since probabilities are attributed to the mechanical states of the total system, they are no longer determined by such mechanical states. Instead, in statistical mechanics, the probabilities are usually determined by means of an ‘ensemble’, i.e., a fictitious collection of replicas of the system in question.