Abstract: In an active system, an actor is able to effect action in a subject system. The actor and the subject system exist in an environment which can impact the subject system. Neither the actor nor the subject system has any control over the environment. The actor includes a model and a processor. The processor is guided by the model. The processor is arranged to effect action in the subject system. The subject system is known by the model. This allows the actor to be guided in its action on the subject system by the model of the subject system. Events can occur in the subject system either through the actions of the actor, as guided by the model, or through actions of other actors, or through a change in state of the subject system itself (e.g. the progression of a chemical reaction) or its environment (e.g. the passage of time). The actor keeps the model updated with its own actions.