With the Stoics ended all inquiry among the Greeks of a philosophical nature worthy of especial mention, until philosophy was revived in the Christian schools of Alexandria, where faith was united with reason. The Stoics endeavored to establish the certitude of human knowledge in order that they might establish the truth of moral principles, and the basis of their system was common sense, with which they attacked the godless skepticism of their times, and raised up a barrier, feeble though it was, to prevailing degeneracy. The struggles of so many great thinkers, from Thales to Aristotle, all ended in doubt and in despair. It was discovered that all of them were wrong, but that their error was without a remedy. [Sidenote: Bright period of Grecian philosophy.] The bright and glorious period of Grecian philosophy was from Socrates to Aristotle. Philosophical inquiries began about the origin of things, and ended with an elaborate systematization of the forms of thought, which was the most magnificent triumph that the unaided intellect of man ever achieved. Socrates founds a school, but does not elaborate a system. He reveals most precious truths, and stimulates the youth who listen to his instructions by the doctrine that it is the duty of man to pursue a knowledge of himself, which is to be sought in that divine reason which dwells within him and which also rules the world. He confides in science; he loves truth for its own sake; he loves virtue, which consists in the knowledge of the good. [Sidenote: Summary.] Plato seizes his weapons and is imbued with his spirit. He is full of hope for science and humanity. With soaring boldness he directs his inquiries to futurity, dissatisfied with the present, and cherishing a fond hope of a better existence. He speculates on God and the soul. He is not much interested in physical phenomena. He does not, like Thales, strive to find out the beginning of all things, but the highest good, by which his immortal soul may be refreshed and prepared for the future life he cannot solve, yet in which he believes. The sensible is an impenetrable empire, but ideas are certitudes, and upon these he dwells with rapt and mystical enthusiasm,--a great poetical rhapsodist like Xenophanes, severe dialectician as he is, believing in truth and beauty and goodness. Then Aristotle, following out the method of _his_ teachers, attempts to exhaust experience, and directs his inquiries into the outward world of sense and observation, but all with the view of discovering from phenomena the unconditional truth, in which he, too, believes. But every thing in this world is fleeting and transitory, and, therefore, it is not easy to arrive at truth. A cold doubt creeps into the experimental mind of Aristotle with all his learning and all his logic.