The need for engineering principles

Prototyping of expert systems provides a means of augmenting the conventional sequential software development life-cycle; it is not a substitute for it. Any prototyping exercise requires a consideration of two levels of system description, namely the prototype system itself and the ultimate desired system whose design alternatives are to be explored by the prototype. Neither system description can be complete in practice since firstly the complete technical description of the ultimate system is impossible without experimentation, and secondly the prototype is an incomplete simulation of the full-scale system. The relationship between the ultimate operational system, sets of prototypes, prototype of prototypes, and so on, need to be managed explicitly in order to ensure that the appropriate conclusions, concerning hardware, software, development methodology are associated with the correct level of specification. The hardware and software tools needed during the prototyping phases of system development, when flexibility of alternative design and implementation is most important, are generally not the same as those required during the design and implementation of the operational system