For five decades complex socio-technical systems have been studied in an attempt to understand and prevent the occurrence of accidents. In this paper, the authors define a concept for a System Complexity metric, comprising the total of all direct interactions between the system elements and the tools the controller has to control the system. Subsequently, the human performance of the operator is taken into account to arrive at the Perceived System Complexity. Finally a hypothesis for a relation between the dynamic actually perceived system complexity, and the occurrence of incidents is postulated, which are still to be proven in practice.