The key idea in any modeling approach that starts from a priori knowledge about the physical behavior of the system to be modeled is that a conceptual separation is made between various
that are (considered to be) relevant in a given problem context. Although this step is always present, it is often preceded by a step in which the system to be modeled is subdivided into subsystems on the basis of aspects of
. Note that within the context of dynamic modeling, and thus in the context of this text, the adjective ‘physical’ (physical system, physical model, physical modeling, etc.) is not used in the sense of ‘concrete’, but in the sense of ‘obeying the laws of physics’. Consequently, a physical model in this sense of ‘obeying the laws of physics’. Consequently, a physical model in this sense can be rather abstract. A pump drive system, for example, can be seen as an interconnection of an electric power source (an amplifier), an electric motor, a transmission (a gear, box) and a mechanism for displacement of a fluid (a load). This is the first conceptual level of observable,
. The total behavior consists not only of the observable dynamic interaction between these subsystems, but also of the conceptual interaction between the fundamental behaviors that constitute the behavior of these subsystems themselves. Already when modeling at this level, it is useful to consider both forms of interaction (observable and conceptual) from the point of view of
), instead of the unidirectional input-output relations that are often used, thus implictly assuming no ‘back-effect’.