Philosophy is an edifice — the highest and most stable edifice — of human reason, which moves from the firm foundation of experience and rises to the ultimate cause to justify that same experience.
In an abstract or absolute sense, philosophy could be realized by healthy and efficient human reason confronted with the riddle of the world and of life, without tortuous twistings and turnings to reach its goal. The history of philosophy shows us, however, that the system of philosophy, as in the case of all great human values, is not obtained save through a gradual, painstaking process. To explain or expound this slow, gradual process through which human thought has constructed its philosophy piece by piece — not always advancing — is the office of the history of philosophy.
The historical development of philosophy has come about particularly through four great successive civilizations: Indian, Classic, Christian, and Modern. These differ from one another in the solutions which they have offered to explain the problem of life.
Indian philosophy has had little or no influence on Western civilization (although this may be changing now), and it is for this reason that most textbooks in the history of philosophy in the Western world begin with the exposition of the story of Classic civilization (history of Greek philosophy) and then pass on to a consideration of Christian and Modern civilization, expounding the principal systems of each.
But it is licit to ask oneself: “From what point should a history of philosophy begin its exposition?” To begin with, absolutely speaking, we should say that the history of philosophy opens with the first appearance of man upon earth. Every man, in whatsoever time and place he lives, cannot but ask himself: “Why do I live on this earth?” and attempt to give some answer to the question. For this reason it is said that man is naturally philosophical, and that he is philosophizing even when he believes that he is not and even when he denies the existence of philosophy.
In an objective exposition of a philosophic system of history, it is necessary to keep in mind the ideas not the men who have conceived them. This is because the object of history is the ideas, the theoretical element, and not the sentiments or moral intentions of the philosophers.
It is useful to call the attention of students who are studying or plan to study the history of philosophy that they should not expect to find actual progress in every unraveling of philosophical thought. It is true that every philosophical system indicates a development, an actuation. But that is not to say that every development and every actuation is for the better. There is also a development toward error, which indicates retrogression rather than progress. Hence it is not necessary to affirm a priori that every system which succeeds another is of necessity better than the preceding. Progress is a fact which must be ascertained a posteriori — by examination and comparison — after one has evaluated the system in question in regard to what is the end of philosophy: the knowledge of absolute reality, the solution of the problem of life.
Finally, let us note that the duty of the historian of philosophy is to reconstruct objectively the thought of the philosopher, whatever it may be, prescinding from its theoretical value or truth. Only after such work is done in reconstruction will it be possible to pass to external criticism. The history of philosophy contains philosophy, but it also contains error; for as has been said before, history does not always relate signs of progress, but often recounts regression as well. History hence should be complemented by an external criticism, which judges the validity or value of every system with reference to the essentially philosophical problem: the value of life.