If the topic be highly abstract, show its nature by concrete...

If the topic be highly abstract, show its nature by concrete examples; if it be unfamiliar, make it figure as part of a story; if it be difficult, couple its acquisition with some prospect of personal gain. Above all things, make sure that it shall run through certain inner changes, since no unvarying object can possibly hold the mental field for long. Let your pupil wander from one aspect to another of your subject, if you do not wish him to wander from it altogether to something else, variety in unity being the secret of all interesting talk and thought.