This fundamental question has assumed three separate forms or aspects in the history of philosophy. These forms have been determined by the objective phenomena which most immediately arrested and engaged the attention of men. If external nature has been the chief object of attention, then the problem of philosophy has been, _What is the ἀρχή--_the beginning; what are the first principles_--the elements from which, the ideas or laws according to which, the efficient cause or energy by which, and the reason or end for which the universe exists?_ During this period reflective thought was a PHILOSOPHY OF NATURE. If the phenomena of mind--the opinions, beliefs, judgments of men--are the chief object of attention, then the problem of philosophy has been, _What are the fundamental Ideas which are unchangeable and permanent amid all the diversities of human opinions, connecting appearance with reality, and constituting a ground of certain knowledge or absolute truth?_ Reflective thought is now a PHILOSOPHY OF IDEAS. Then, lastly, if the practical activities of life and the means of well-being be the grand object of attention, then the problem of philosophy has been, _What is the ultimate standard by which, amid all the diversities of human conduct, we may determine what is right and good in individual, social, and political life?_ And now reflective thought is a PHILOSOPHY OF LIFE. These are the grand problems with which philosophy has grappled ever since the dawn of reflection. They all appear in Greek philosophy, and have a marked chronology. As systems they succeed each other, just as rigorously as the phenomena of Greek civilization. The Greek schools of philosophy have been classified from various points of view. In view of their geographical relations, they have been divided into the _Ionian_, the _Italian_, the _Eleatic_, the _Athenian_, and the _Alexandrian_. In view of their prevailing spirit and tendency, they have been classified by Cousin as the Sensational, the Idealistic, the Skeptical, and the Mystical. The most natural and obvious method is that which (regarding Socrates as the father of Greek philosophy in the truest sense) arranges all schools from the Socratic stand point, and therefore in the chronological order of development: I. THE PRE-SOCRATIC SCHOOLS. II. THE SOCRATIC SCHOOLS. III. THE POST-SOCRATIC SCHOOLS.