It has been proposed to integrate psychotherapies on the basis of general therapy change principles (TCPs). In this theoretical study, the authors analyzed the concept of psychotherapy and evaluated the TCPs' integrative approach from the position of this concept. The authors found that the TCPs' conceptualization is largely inconsistent with the concept of psychotherapy and contains assumptions that either raise conceptual issues or do not appear to fit with clinical evidence. In contrast to what is asserted or implied in the TCPs' approach, the abstract nature of the change principles means that they do not denote causal relationships, are not empirically testable, and cannot serve as a useful guide for clinical action. Moreover, the embeddedness of therapy procedures in causal schemas makes the assumption that therapists can master a multitude of procedures from different therapies unrealistic. Also against the assumptions of the TCPs' approach, the total effect of therapy is probably not the sum of the effects of the procedures and procedures and rationales from different therapies cannot be combined. Overall, the issues of the TCPs' integrative approach seem to stem from the omission of the essential feature of psychotherapy: to operate according to causal schemas of psychological phenomena. The authors argue that a focus on differences between therapies instead of common principles may lead to important assimilative integrations.