Plato went to Sicily in order to assist in a political experiment, and throughout his life studied mathematics. In those days mathematics and its applications were not so separated as they can be today. No doubt, the sort of facts that he observed were the applications of mathematical theory. But no one had a keener appreciation than Plato of the divergence between the exactness of abstract thought and the vague margin of ambiguity which haunts all observation. Indeed in this respect Plato, the abstract thinker, far surpasses John Stuart Mill, the inductive philosopher. Mill in his account of the inductive methods of science never faces the difficulty that no observation ever does exactly verify the law which it is presumed to support. Plato’s feeling for the inexactness of physical experience in contrast to the exactness of thought certainly suggests that he could look for himself. Mill’s determinism is, according to his own theory, an induction respecting the exactness of conformation to the conditions set by antecedent circumstances. But no one has ever had any such experience of exact conformation. No observational basis whatsoever can ever be obtained for the support of Mill’s doctrine. Plato knew this primary fact about experience, Mill did not. Determinism may be the true doctrine, but it can never be proved by the methods prescribed by English empiricism.
When we come to Aristotle the enumeration of his practical activities makes us wonder that he had any time for thought at all. He analyzed the consti-
