The paper aims to propose a formalization of the concept of ceteris paribus (CP) by means of a dynamic model. The basic result of the analysis is that the CP clause may assume essentially different meanings according to (1) the kind of variables assumed to be ‘frozen’ and (2) the length of the time horizon. It is then possible to distinguish, respectively, between an exogenous and an endogenous CP and, within the latter, between a short-run and a long-run CP. This double analytical distinction helps in understanding the role the CP clause plays in Marshall's thought and in economics in general.