After having underlined the ambiguities of the concept of plasticity and the dangers of its purely metaphoric use in neurobiology, it is suggested that we return to a more precise definition of the structure, the operating principles and the function of the "systemic" unit or "integron" relevant to the particular level of analysis in question. Any change can then be described as a modification of function, a change in the operation principles, or an alteration of the material structure of the system. It is suggested that the term plastic should be restricted to describing, among the possible variations in the operating principles or the function of a given system, any lasting alteration of the connectivity network of the system under the influence of an external force or environmental constraint. Therefore, systematic or random variations of performance, functional flexibility or the vicarious(1) processes or strategies that can be found in a rigidly wired system are not justified examples of plasticity.