Types of hierarchy imply types of model.

The types of hierarchy that describe the knowledge structures used by people doing complex tasks imply that certain types of cognitive mechanism are available to process them. This paper will discuss principles of organization in is-a (classification) and part-whole hierarchies. The principles of organization may be different at different levels of a hierarchy. The behaviour of people doing complex tasks is adaptable as a function of context. These factors taken together imply that cognitive processing is done by independent modules working within a context, rather than by a sequence of processing stages, and that a sequential stages processing model is a special case of a more general process. The paper ends with some practical implications of this change in models of the cognitive processes underlying complex behaviour.