1. A perceptual learning system for teaching students to recognize diagnostic structure from varying instances, the system comprising: a microprocessor; a memory accessible by the microprocessor; an interface coupled to the microprocessor for interacting with a student; a problem database in the memory including: a plurality of learning items which are examples of different concepts including at least one given concept to be learned by the student, the different concepts being categories of structure that are distinguishable from one another by perceptual structure; with respect to the concept to be learned, a plurality of learning items that are positive instances of the concept to be learned containing diagnostic structural elements sufficient to determine the concept to be learned that can be perceived by the student and a plurality of negative instances which do not contain diagnostic structural elements sufficient to determine the concept to be learned; and with respect to the positive instances of the concept to be learned, a plurality of learning items corresponding to different examples of the concept to be learned that vary in attributes other than the diagnostic structural elements sufficient to determine the concept to be learned; wherein the microprocessor is configured to present, in a presentation sequence and with a presentation delay interval, the learning items to the student as a series of learning trials, the series of learning trials including positive instances and negative instances of the concept from the problem database; wherein the microprocessor is configured to vary the set of positive instances presented to the student in the series of learning trials, by retrieving and presenting to the student over the series of learning trials multiple different examples of positive instances of the concept to be learned that vary in irrelevant attributes other than the diagnostic structure sufficient to determine the concept, and wherein the microprocessor is configured to receive a response from the student on at least some of the learning trials presented to the student; and a delay counter for counting number of learning trials for each learning item presented to the student since last presentation of said each learning item; wherein the microprocessor is configured to monitor the responses received from the student and determine a response speed and a response accuracy of the student's response to one or more of the learning trials; wherein the microprocessor is configured to vary the presentation sequence of the learning items presented to the student relating to the concept to be learned based upon the determined response speed and response accuracy, and wherein the microprocessor is configured to vary the presentation delay interval for a learning item, based on the number of learning trials counted by the delay counter, the determined response speed, and the determined response accuracy.