It is argued that the engineering discipline of human-computer interaction (HCI) has developed over the last couple of decades in an ad hoc manner, driven by the need to solve real problems, rather than being informed by high level, general theories of the discipline. A retrospective role for general HCI theories is suggested. A start at such a general theory, which must be simple and able to encompass all the discipline of HCI's activities, is proposed, based on the concept of tasks. Tasks are characterised as the means by which work is performed. A general systems modelling approach is introduced which divides the assumed world it models into work systems and the application domains that are changed by work performance. The role of different work systems, defined by their differing boundaries and goals, to define different subtasks is introduced and illustrated with a number of simple examples.