"In attempting to teach the prince how to achieve, maintain, and expand power, Machiavelli made his fundamental and celebrated distinction between "the effective truth of things" and the "imaginary republics and monarchies that have never been seen nor have been known to exist." The implication was that moral and political philosophers had hitherto talked exclusively about the latter and had failed to provide guidance to the real world in which the prince must operate. This demand for a scientific, positive approach was extended only later from the prince to the individual, from the nature of the state to human nature. Machiavelli probably sensed that a realistic theory of the state required a knowledge of human nature, but his remarks on that subject, while invariably acute, are scattered and unsystematic."