An investigation of a design project is presented with concepts and theory developed within critical psychology in order to understand the process. Investigating the co-operation of the design of a building allows us to see that the building, its design, and its use constitute common contradictory causes. In their co-ordinated activities, professionals participate from each of their perspectives with different functions and priorities, and in their acts they must take the contradictions into account. Contradictions as incompatible aspects in things and activities make it difficult to keep the common cause stable. This means that the participants’ co-operation is a continuous attempt to produce stability. They are constantly confronted with the possibility of reorganization, development, and conflict in their activities. Therefore, co-operation is conflictual. The theoretical position is based on the fact that our understanding is reciprocally formed with our local, common activities. The theoretical consequences of the position are explored.