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impression, and that therefore we have no other standard of action than utility for the individual. I
A new period of philosophy opens with the Athenian socrates (469-399). Like the Sophists, he rejected entirely the physical speculations in which his predecessors had indulged, and made the subjective thoughts and opinions of men his starting point; but whereas it was the thoughts and opinions of the individual that the Sophists took for the standard, Socrates endeavoured to extract from the common intelligence of mankind an objective rule of practical life. For this purpose he employed the two forms of philosophical inquiry of which he is the inventor, induction and definition. Such a standard he saw in knowledge, by which term he understood the cognition in thought of the true concept of an object, and identified it with Virtue; that is to say, such action as proceeds from clear cognition of the concept appropriate to the circumstances. Thus, although Socrates did not himself succeed in establishing a genuine ethical principle, he is nevertheless the founder of ethics, as he is also of dialectic, the method of the highest speculative thought. Of Socrates' numerous disciples many either added nothing to his doctrine, or developed it in a one-sided manner, by confining themselves exclusively either to dialectic or to ethics. Thus while the Athenian xenophon contented himself, in j a series of writings, with exhibiting the portrait of his master to the best of his comprehension, and added nothing original, the Megarian school, founded by EucLlDES of MegSrS, devoted themselves almost ' entirely to dialectic investigation ; whereas ethics preponderated both with the Cynics and Cyrenaics, although the position taken up by these two schools was in direct antithesis. For antisthenes of Athens, the founder of the Cynics, conceived the highest good to be the virtue which spurns every enjoyment; while aristippds of Gyrene, the founder of the Cyrenaics, considered pleasure to tie the sole end in life, and regarded virtue as a good only in so far as it contributed to pleasure.
Both aspects of the genius of Socrates were first united in plato of Athens (428-348), who also combined with them all the i principles established by earlier philosophers, in so far as they had been legitimate, and developed the whole of this material into the unity of a comprehensive system. The groundwork of Plato's scheme, though
nowhere expressly stated by him, is the threefold division of philosophy into dialectic, ethics, and physics ; its central point is the theory of ideas. This theory is a combination of the Eleatic doctrine of the One with Heraclitus' theory of a perpetual flux and with the Socratic method of concepts. The multitude of objects of sense, being involved in perpetual change, are thereby deprived of all genuine existence. The only true being in them is founded upon the ideas, the eternal, unchangeable (independent of all that is accidental, and therefore) perfect types, of which the particular objects of sense are imperfect copies. The number of the ideas is defined by the number of universal concepts which can be derived from the particular objects of sense. The highest idea is that of the Good, which is the ultimate basis of the rest, and the first cause of being and knowledge. Apprehensions derived from the impressions of sense can never give us the knowledge of true being; i.e. of the ideas. It can only be obtained by the soul's activity within itself, apart from the troubles and disturbances of sense; that is to say, by the exercise of reason. Dialectic, as the instrument in this process, leading us to knowledge of the ideas, and finally of the highest idea of the Good, is the first of sciences, scientia sclentiarum. In physics, Plato adhered (though not without original modifications) to the views of the Pythagoreans, making Nature a harmonic unity in multiplicity. His ethics are founded throughout on the Socratic; with him too virtue is knowledge, the cognition of the supreme idea of the Good. And since in this cognition the three parts of the soul, cognitive, spirited, and appetitive, all have their share, we get the three virtues, Wisdom, Courage, and Temperance or Continence. The bond which unites the other virtues is the virtue of Justice, by which each several part of the soul is confined to the performance of its proper function. The school founded by Plato, called the Academy, from the name of the grove of the Attic hero Academus, where he used to deliver his lectures, continued for long after. In regard to the main tendencies of its members, it was divided into the three periods of the Old, Middle, and New Academy. The chief personages in the first of these were speusippus (son of Plato's sister), who succeeded him as the head of the school (till 339), and xenocrates of Chalcedon (till