1. An actor for effecting action in a subject system, the actor comprising: a model of the subject system, the model including: objects which represent objects within the subject system, each object in the model having associated therewith data which define which of at least two states the object is in at a given time, and rules which define actions which are to be initiated in response to events, an event being the change in the state of an object, and a processor, arranged: to respond to an event in the subject system by initiating at least one action in the subject system which is dependent on the event, as defined by the rules of the model.