1. A perceptual learning system for teaching students to recognize diagnostic structure from varying instances, the system comprising: a problem database containing a plurality of learning items which are examples of different concepts, which, with respect to any one concept, includes a number of positive instances containing diagnostic structural elements and a number of negative instances which do not contain the diagnostic structural elements of the concept; and software for implementing a trial loop, wherein the learning items are presented to the student in the form of classification trials.