Design science is slowly but surely establishing itself as a recognized paradigm for conducting research in information systems. Researchers in the IS field have tried to study different aspects of design science. So far, it seems that the design activity, or "design reasoning" has not received much attention from the IS community. In this paper, we propose to use a theory developed in engineering fields in order to solve this issue. The C-K theory, or Concept-Knowledge theory, is considered to be a good candidate to deal with the design reasoning.