Our complex societies are approaching the limits of educability for large numbers of citizens (e.g. “functional” literacy hovers at around 50% in most “advanced” societies). Even those selected in schools for high intelligence (e.g. many doctors) seem unable to absorb, retain and use critically all of the knowledge necessary to practice their professions competently. With less able students, good teachers often realize that there comes a point of diminishing returns for teaching effort, so difficult choices have to be made.