Abstract: A computer-aided design system simulates the designer thought processes by the provision of a plurality of discrete unit programs provided respectively for individual design items, Each of the unit programs includes a thought process portion, a knowledge portion and a recognition portion. The thought process portion is for receiving an input instruction via an input device, executing logic operations in accordance with the input instruction to obtain logic results, and transferring the logic results to a related unit program. The related unit program receives the logic results and executes logic operations in accordance with the received logic results to obtain new logic results. The knowledge portion of each unit program is provided for storing the various logic results, and the recognition portion of each unit program is provided for identifying the related unit program which is related to the logic results obtained by executing the logic operations.