1. A computer-aided design system comprising: an input device; a display device; and, processing means including a plurality of discrete unit programs which are operatively independent from each other and which are provided respectively for discrete design items, each of the plurality of discrete unit programs including a thought process portion, a knowledge portion and a recognition portion, wherein said thought process portion of each unit program includes means for receiving at least one of an input instruction via said input device and logic results from another unit program, executing logic operations when received logic results are associated with said logic operations in accordance with the input instruction to obtain logic results and transferring the logic results to all of a plurality of related unit programs, wherein said knowledge portion of each unit program includes means for storing the logic results, and wherein said recognition portion of each unit program includes means for identifying the plurality of related unit programs which are related to the logic operations of said each unit program; said processing means, responsive to an input user instruction, for processing a first discrete unit program associated with a user selected design item to cause the first discrete unit program to obtain first logic results and identify a plurality of related second discrete unit programs and transfer the first logic results to all of the related second discrete unit programs, processing all of the related second discrete unit programs to determine at least one related second discrete unit program which is for executing logic operations associated with the transferred first logic results, and for then activating in succession the at least one related second discrete unit program having logic operations associated with the transferred first logic results and any subsequent related discrete unit programs identified during processing of a preceding discrete unit program.