A distinction is drawn and discussed between two modes of computer use: as a tool and as an instrument. The former is typical for the use of a conventional software product, the latter is more appropriate in volatile environments or where close integration of human and computer processes is desirable. An approach to modelling developed at Warwick and based upon the concepts of observable, dependency and agency has led to the construction of open-ended computer-based artefacts called 'interactive situation models' (ISMs).
The experience of constructing these ISMs, and the principles they embody, exemplify very closely the characterisation of instruments as 'maintaining a relationship between aspects of state'. The framework for modelling that we propose and report on here seems well-suited to account for the dual 'tool-instrument' use of computers. It is also sufficiently broad and fundamental to begin the deconstruction of human-computer interaction that is called for in any attempt to understand the implications of computer-based technology for human cognitive processes.