The personal contacts between Aristotle and Plato, those of a great disciple and a great teacher marked by mutual cool respect, came to an end with Plato's death. However, Plato's ideas were ever present in Aristotle's mind. His own thought was governed by some of them, others he rejected for particular reasons. The rise of Aristotle's philosophy alongside Plato's philosophy is an example of the controversial split in the development of thought in one and the same era; if we regard Plato and Aristotle as representatives of two different, correlative and still live types of philosophers, we realise that after Plato Aristotle had to come, sooner or later, so that in Greek philosophy both main schools of philosophic investigation might be expressed. Aristotle is for human thought an indispensable complement of Plato and the knowledge of Plato is an introduction necessary to an understanding of Aristotle's philosophy.¹