In this paper it is argued that methods are needed for the design of a larger variety of product aspects than is feasible with mechanical engineering design methodology of today. Design methods found within the European schools of design are inadequate for the design of products other than machine systems of transforming character. The reason for this is that the underlying theories only describe the nature of ‘operand-transforming’ technical systems, and that the description of the process and function systems are too narrowly defined to be useful for the design of ‘non-transforming’ products, or for products where the human is involved as an active user.
The paper takes as the standpoint that the functional language, in accordance with established foundation in engineering design theory, is a successful means to treat usability aspects of human-product systems. An extended process modeling view based on product life-phase thinking including a ‘use-process’ is presented, focusing the attention towards the use, and not merely the workings, of the product. Also, extended definitions of a number of concepts are proposed, and function-classes of the human-product system, leading to a more generally applicable use of functions as a modeling tool when describing products, is introduced. The proposed functional language is illustrated in a product case example.