We shall say that a model G (for general) is more general than a model S (for special) if the set of conditions in which model G is applicable is a superset of the conditions of applicability of model S. A condition of application is positive if it is the specification of circumstances in which a model is applicable or negative if it specifies circumstances in which a model is not applicable. Consequently, a model G will be more general than a model S if the negative conditions of application of the former are a subset of the negative conditions of application of the latter and the positive conditions of application model G are a superset of those of model S and either the negative conditions are a proper subset or the positive conditions a proper superset. In all other cases, the hierarchy of generality is ambiguous.
Sometimes, generality is associated with simplicity. This would be the case if model S had extra conditions of application as well as all of the conditions of application of model G. An example of simplification implying generalisation will be explored in the companion to this paper (part 2).