On the conceptual framework of John Gero's FBS-model and the prescriptive aims of design methodology

In this paper we consider the Function–Behaviour–Structure model of designing in the version as developed by John Gero and collaborators. We identify two problems: the absence of a stable definition of function, and the model's double aim of describing actual designing and prescribing improved designing. These problems are not unique to the body of work we are addressing here; they are general problems that design methodologists have been struggling with in the last forty years. We argue that philosophy may help addressing them: it may fix a definition of function and illustrates how descriptive and prescriptive modelling can be distinguished and connected.