The new generation of medical knowledge-based systems (KBS) will establish a kind of colleagueship between intelligent computer agents and physicians. Each will perform the tasks that it, he or she does best, and the intelligence of the system will be an emergent of the collaboration. The goal is to build mental prostheses that help physicians with different skills and expertise in the management of patients. Just as telescopes are designed to extend the sensory capacity of humans, KBS are designed to extend their cognitive capacity.