This paper is a charter presenting a scientific posture shared by signatories in the use of simulation tools when dealing with complex systems. This posture is based on a cycling approach, in interaction with field processes, including discussion of assumptions and feedbacks on the field process. Confrontation between field and modelling processes has to be permanent because of openness and uncertainty features of these systems. This approach is used with two possible aims: learn on systems or support collective decision processes in these systems, which corresponds to an objective of increasing knowledge either for the scientist or the field actors, always through an interaction between them mediated by an evolutionary model. Both aims lead to different implementations of this companion modelling approach, but each one is side effect of the other one, and has to be taken in account as such. Scientists ready to work in that way and make this posture alive are kindly invited to join.