The earliest Indian philosophy is to be found in the Upanisads, and the Upanisads have a distinct mystic and religious strain. From this fact we should not, like Zeller or Erdmann, draw the erroneous conclusion that early Indian philosophy should be excluded from an account of empirical or rational philosophy. It is true that so long as mysticism is the experience of an individual, we cannot apply to it the tests of philosophical inquiry. But when an attempt is made to build up a logical system of speculation on the basis of such experience, it must not only be included within the province of philosophy but may well constitute an important part of it. If we do not apply to it the name of philosophy. There is hardly any other term which can describe it.
What is philosophy? Philosophy is an inquiry into the nature of life and of existence. We have two ways of dealing with reality. One starts and ends with revelation and tradition; we call it religion. The second depends on the free exercise of reason and thought and is called philosophy. Philosophical inquiry from the earliest times has adopted one of two alternate ways of approaching its problems. One is through the world of man's inner being and the other through the world external to him.
The characteristic of Indian thought is that it has paid greater attention to the inner world of man than to the outer world. It does not begin with an investigation into outer phenomena and reach towards the inner reality. On the contrary, it starts from the realization of the inner world and reaches out to the world of phenomena. It was this way of approach that revealed itself in the philosophy of the Upanisads.
In Greece also, the earlier Schools of philosophy had adopted a similar procedure or at least it was not excluded from their general approach. That we know of the Orphic or the Pythagorian philosophy tends to support this statement. The dialectical method of Socrates was, no doubt, logical, but he declared that he was guided by an inner voice. Like Indian philosophy, the message of some Greek philosophers also was "Know thyself."
In Platonic idealism we find the germs for the future development of mysticism, as well as of the knowledge of the inner self, but his disciple, Aristotle, did not choose to develop either of these lines of thought. Ultimately, however, mysticism came to fruition in Alexandria and culminated in the philosophy of Neo-Platonism. We cannot say definitely whether the Upanisad philosophy of India was responsible for the development of this Alexandrian School. We, however, know that Alexandria had in that era become the meeting-place for the religions and civilizations of the East and the West. Just as gods of different religions had met in its market-place and led to the foundation of the Serapeum, it seems probable that the different streams of human thought and inquiry met here and mingled in one common flow.
What is the basic principle of mysticism? It is that the knowledge of reality cannot be obtained through the senses. If we are to reach reality, we must withdraw from the world of sense into that of inner experience. This principle, in some form or other, worked in the philosophical systems from Pythagoras to Plato. Plato made a sharp distinction between the world of thought and the world of sense. He expressed their difference by the analogy of the distinction between the light of midday and twilight. According to him, whatever we perceive through the senses is perceived as in twilight. What we perceive through the intellect is seen in the clear light of day.
He emphasizes, again and again, the distinction between Appearance and Reality. The senses can reach us only up to the world of Appearance but not to the world of Reality. He expresses the ultimate real as the Good. Science, knowledge and truth deal with Ideas which are like the Good, but it is only the Good that is ultimately real. We cannot reach the Real through the mediation of sense. The famous parable of the cave-dwellers which he relates in The Republic is the final statement of his philosophy. Though he does not speak of intuitive reason on which Upanisadic Philosophy is based, the way in which he repudiates objects of experience given through sense perception brings him very near the attitude of the mystics towards the world of sense.
There is also a second similarity between Indian and Greek philosophy which should not be overlooked. The concept of “nous” in Greek philosophy is not very dissimilar to that of “atman” in Indian philosophy. Plato rejected the views of Anaxagoras and distinguished between two souls. He regards one as immortal and the other as mortal. The mortal soul (irrational soul) is not free from the influence of the body and maybe called the ego. The immortal soul is the Idea of the Universe and is free from all influences of the body. This immortal soul is called by him "Universal Soul." If therefore we try to contrast Plato's concept of the mortal soul with that of the immortal soul, it will not be very different from the contrast between “jivatman” and “paramatman” of Indian philosophy.
It will not therefore be proper to exclude Upanisadic philosophy from a general account of philosophy on the ground that it is mystic. If we do so, we would also have to exclude a major portion of Greek philosophy from any such general account.
We must also remember that what differentiates philosophy from what is non-philosophy is not difference of subject-matter but of method and treatment. If a person's conclusions rests upon the authority of revelation or on individual ecstasy, we would more properly describe his findings as theology or mysticism and not philosophy. If, however, he adopts a method of intellectual construction and considers that the mystery of existence must be solved on the rational plane, we cannot exclude him from the rank of philosophers even though religious or mystic beliefs may have influenced him. Actually, some of the most important material of philosophy is derived from such discourses.
In Christianity and Islam, there developed certain Schools which sought to subordinate philosophy to religion. But their own discourses have by general consent been included among philosophical writings. The reason for this is that they sought to defend religion against rationalist attacks by the use of rationalist methods. The discourses of St. Augustine and the later Christian scholastics cannot therefore be excluded from philosophical literature. The same remark applies to the writings of the Muslim scholastics. So far as Arab philosophy is concerned, one of the schools of which it can justly be proud will be excluded if we leave out this scholastic literature.
Among the Arab philosophers the names of Ibn Sina (Avicenna) and Ibn-al-Rushd (Averroes) are well known, but they were not spokesmen of Arab philosophy proper. They were in fact followers and commentators of Aristotle. If we want to inquire into Arab philosophy proper, we must turn our eyes from them and study the writings of the scholastics who were often regarded as the antagonists of Greek philosophy. It is interesting to note that in modern times Bishop Berkeley, who embarked on philosophical speculations in order to establish the truth of religion, has been always counted among the philosophers and no history of philosophy is complete without an account of his writings.
Nor is Zeller's criticism that "Indian philosophy never lost contact with religion and never became independent" justified. He perhaps had in mind the veneration in which the Vedas were generally held, but he was probably not aware that there were at least three unorthodox schools that repudiated the authority of the Vedas. Neither Buddhism, nor Jainism, nor Charvaka philosophy depends on authority or tradition for its findings. Not only so, but even among the orthodox schools Nyaya and Samkhya, philosophies often paid only lip service to the authority of the Vedas. We may therefore safely say that Indian philosophy had in the age of the Buddha already established a position independent of religion.