But man cannot live without some guiding rule. If his speculations in Nature will yield him nothing on which he may rely, he will seek some other aid. If there be no criterion of truth for him in philosophy, he will lean on implicit, unquestioning faith. If he cannot prove by physical arguments the existence of God, he will, with Socrates, accept that great fact as self evident and needing no demonstration. He will, in like manner, take his stand upon the undeniable advantages of virtue and good morals, defending the doctrine that pleasure should be the object of life--pleasure of that pure kind which flows from a cultivation of ennobling pursuits, or instinctive, as exhibited in the life of brutes. But when he has thus cast aside demonstration as needless for his purposes, and put his reliance in this manner on faith, he has lost the restraining, the guiding principle that can set bounds to his conduct. If he considers, with Socrates, who opens the third age of Greek development--its age of faith--the existence of God as not needing any proof, he may, in like manner, add thereto the existence of matter and ideas. To faith there will be no difficulty in such doctrines as those of Reminiscence, the double immortality of the soul, the actual existence of universals; and, if such faith, unrestrained and unrestricted, be directed to the regulation of personal life, there is nothing to prevent a falling into excess and base egoism. For ethics, in such an application, ends either in the attempt at the procurement of extreme personal sanctity or the obtaining of individual pleasure--the foundation of patriotism is sapped, the sentiment of friendship is destroyed. So it was with the period of Grecian faith inaugurated by Socrates, developed by Plato, and closed by the Sceptics. Antisthenes and Diogenes of Sinope, in their outrages on society and their self-mortifications, show to what end a period of faith, unrestrained by reason, will come; and Epicurus demonstrated its tendency when guided by self. Thus closes the third period of Greek philosophical development. [Sidenote: Age of Reason--its solutions.]