On the conceptual framework of John Gero’s FBS-model and the prescriptive aims of design methodology Pieter E. Vermaas, Department of Philosophy, Delft University of Technology, Jaffalaan 5, 2628 BX Delft, The Netherlands Kees Dorst, Faculty of Design, Architecture and Building, Sydney University of Technology, Australia Department of Industrial Design, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands In this paper we consider the FunctioneBehavioureStructure 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. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: design methodology, design models, design knowledge, descriptive and prescriptive modeling, philosophy of design I n this paper we describe and critically consider the FunctioneBehavioure Structure model of designing in the version as developed at the Key Centre of Design Computing and Cognition at the University of Sydney, under the supervision of John Gero. We identify two fundamental problems. The first concerns the conceptual framework of the model and consists of the absence of a stable definition of in particular the key concept of function. The second is the model’s double status as a descriptive model aimed at describing actual designing and as a prescriptive model aimed at improving designing. The FBS-model seems untenable as a description of actual design cases, sug- gesting that some of its descriptive claims are prescriptive only, and raising questions about how its prescriptive claims are related to its (remaining) de- scriptive claims. In the constructive part of this paper we argue that philosophy may help in addressing these problems. We present philosophical work on function theory and show that it may fix a useful definition of function. And we present Corresponding author: Pieter E. Vermaas p.e.vermaas@tbm. tudelft.nl www.elsevier.com/locate/destud 0142-694X $ - see front matter Design Studies 28 (2007) 133e157 doi:10.1016/j.destud.2006.11.001 133 Ó 2006 Elsevier Ltd. All rights reserved. Printed in Great Britain