Perhaps his doctrine of the necessity of a final cause, or sufficient reason, which gives a rational explanation of individual {224} things, is Aristotle's greatest contribution to pure philosophy. The doctrine of empiricism has been ascribed to Aristotle, but he fully recognized the universal, and thought it connected with the individual, and not separated from it, as represented by Plato. The universal is self-determining in its individualization, and is, therefore, a process of identification rather than of differentiation. The attention which Aristotle gave to fact as opposed to theory, to investigation as opposed to speculation, and to final cause, led men from a condition of necessity to that of freedom, and taught philosophers to substantiate their theories by reason and by fact. There is no better illustration of his painstaking investigation than his writing 250 constitutional histories as the foundation of his work on "Politics." In this masterly work will be found an exposition of political theories and practice worthy the attention of all modern political philosophers. The service given by Aristotle to the learning of the Middle Ages, and, in fact, to modern philosophy, was very great. Aristotle was of a more practical turn of mind than Plato. While he introduced the formal syllogism in logic, he also introduced the inductive method. Perhaps Aristotle represented the wisest and most learned of the Greeks, because he advanced beyond the speculative philosophy to a point where he attempted to substantiate theory by facts, and thus laid the foundation for comparative study. _Other Schools_.--The Epicureans taught a philosophy based upon pleasure-seeking--or, as it may be stated, making happiness the highest aim of life. They said that to seek happiness was to seek the highest good. This philosophy in its pure state had no evil ethical tendency, but under the bad influences of remote followers of Epicurus it led to the degeneration of ethical practice. "Beware of excesses," says Epicurus, "for they will lead to unhappiness." Beware of folly and sin, for they lead to wretchedness. Nothing could have been better than this, until people began to follow sensuality as the immediate return of efforts to secure happiness. Then it led to {225} corruption, and was one of the causes of the downfall of Greek as well as the Roman civilization.