Thus, at least in intent, they are normative methods-they prescribe how design decisions should be made. The validation of a normative theory is a mathematical exercise. It begins with the statement and validation through consensus of a set of axioms. This set of axioms comprises a belief system within which the structure of a design logic must exist. From axioms, one derives theorems.
Theorems are provable within the context of their supporting axioms.
Then, a design method can be validated by deriving it from the resulting theorems, or invalidated by showing that it is inconsistent with the theorems. Thus, the importance of an axiomatic basis for design is clear. Without such axioms, we have no basis thinking that any design method is valid, we have no means of validating a design method, and we have no means of deriving a valid design method. But, a set of axioms that is agreed upon, together with resulting theorems that enable their application, comprises a theory of design. Therefore, a theory of design is necessary.