# An Extended Functional Design Approach Based on Working Principle and Property and Its Application
## 3 Working Principle and Property  
In affordance design theory [28], design represents the specification of a system structure that possesses certain desired affordances to support certain desired behaviors. It is a system structure that determines the affordances describing what behaviors are possible. By changing the structure of a system, designers can change the system's affordances. However, there are still some obvious challenges in implementing affordance-based design and identifying affordances. From the perspective of affordance-based design or function-based design, the importance of structure has never changed. Therefore, how to generate a feasible structure for the desired function or affordance is still crucial and necessary.  
[PERSON] (2009) [29] believed that novel technologies or inventions use new principles. A principle is a concept, the idea of some effect or phenomenon in use. Inventions arise from linking the needs of some purpose with an exploitable effect (or set of effects) conceptually and in a physical form. He pictured this linkage as a chain. At one end of the chain is the need or purpose to be fulfilled; at the other is the base effect that will be harnessed to meet it. Linking the two ends is the overall solution, the new principle, or the concept of the effect to accomplish that purpose. Invention falls into two broad patterns: it may start from one end of the chain, from a given purpose or need, and find a principle to accomplish this; it may start from the other end, from a phenomenon or effect, usually a newly discovered one, and envisage some principle of use in it. From this viewpoint, conceptual design is similar to invention. Meanwhile, designers are required to find suitable principles to realize the desired functions. Compared to the existing discussion on principles, e.g., the physical law in General Design Theory (GDT) ([PERSON] and [PERSON] 1985, 1994) [30; 31], the physical effect in engineering design ([PERSON] and [PERSON], 2007) [3], and the abstract principle in NFPS ([PERSON], et al., 2015) [23; 24], some other necessary knowledge besides the idea of effect or phenomenon is still not considered. Therefore, the working principle is employed to integrate the necessary principle knowledge involved in functional design.  
A working principle is a type of abstract knowledge resource that needs to be transformed into a single or a very small number of physical structures (whether conceptual or materialized). An intermediate transformation variable is still lacking. This variable can establish a direct mapping relationship with the working principle, such as Hooke's law and the elastic coefficient. The variable can also be an inherent property or designed property of a specific physical structure. The current understanding of \"function to structure\" does not realize this linkage. Therefore, the property is also adopted as the intermediate transformation variable between the working principle and the structure.  
Based on the above discussion, the following content of this section will focus on the working principle, property, and structure.