An optimal action generation engine, called an actor for short, interacts with an application program or the environment by means of exchanging signals. The actor emits output signals (or action signals) on the basis of input signals supplied to it. A list of occurrences of input and output signals with particular moments of time when those occurrences took place represents the event history of the actor.
The actor generates output signals according to spur increments, which belong to one or more spur types and are supplied to it along with input signals. See Spur-driven Behavior, for definitions of spur and spur type.
To every spur type there correspond a way of spur perception, spur weight, and a type of time used to compute spur increment velocity. A list of definitions of spur types used by the actor specifies its spur scheme.