For Plato an important, perhaps man’s most important, intellectual task was to distinguish appearance from reality. It is a task required not only of the contemplative philosopher or scientist but, even more, of the man of action, in particular the administrator or ruler, who has to find his bearings in the world of appearance and who must know what is the case, what can be done, and what ought to be done. To achieve order, theoretical or practical, in the world of appearances, which is always changing, we must know the reality, which never changes. Only in so far as we know that, can we understand and dominate the world of appearance around us. (S. Körner, The Philosophy of Mathematics: An Introductory Essay (London: Hutchinson & Co., 1960), p. 14)