The Upanishads, in which those truths were expressed, are sometimes called secret teachings, and no doubt the word upanisad has some such implication not secret in the sense of mysterious, but in the sense that these truths are not found on the surface by the average mind; they are buried deep down and have to be discovered by everyone in his inmost being. Further, when these teachings were given to a pupil, the pupil approached the teacher and sat near him, and the teacher gave this teaching to him alone, not in the presence of others. Even now, these teachings are given in private. Others are not allowed to be present, because it is considered that anything given out publicly can never take root in the deep life of a person. Just as the roots of a plant generally die when they are exposed to the sun or the outside atmosphere, in the same way whenever you express something it fails to go deep into your life, and you hate therefore to speak in public of the deepest things; they should be kept hidden within. On this psychological fact the tradition of privacy was built.
Now, as I said, it probably took two thousand years to develop and consolidate these teachings; that is the orthodox Hindu belief. Many would not agree with it, but when I consider how long it takes to find one single truth, and when I remember that the truths expressed in the Upanishads were not inherited by these people but had to be discovered by them when I consider these facts,
I cannot but think that the orthodox Hindu belief is correct.
As I said, the principles of Vedanta are universal. What are these principles about? Because they are concerned with ultimate reality, they are about anything you would call real the world, for instance. And certainly, I, for myself, am real, and I want to know the last word about myself; so the soul or the Self is discussed. And since there was a belief in an ultimate Being, there was a great deal of speculation about God. In other words, Vedanta considers the same problems that every other philosophical system considers God, soul, and the world. Along with that, many other things are involved. For example, Vedantists have given a great deal of thought and time to the discussion of different states of mind or of consciousness, which they found very important for the determination of truth. Other topics were also discussed, some that have no real significance for us today, and some that can help us a great deal. Of course, much has probably been lost, but enough is left.
It is said that there are altogether a hundred and eight Upanishads. It is quite obvious that most of these are not true Upanishads at all; that is to say, they did not form a part of the original Vedas but were written afterwards. From that you should not conclude that they are worthless; as a matter of fact, some are highly illuminating and explain many things not found in the original Upanishads, which some scholars have said number twenty-eight. Of these twenty-eight, some say twelve and others say ten are the principal ones. Shankaracharya, to whom we owe the revival of Vedanta after Buddhism degenerated, commented upon ten Upanishads, and therefore many think that these ten must have been the most authoritative. Two of them the Chandogya and the Brhadaranyaka are very large and in many places very abstruse. In fact, one is compelled to confess that some passages cannot today be explained at all. Other Upanishads are smaller; some are composed of just a few verses. But all of them have been considered of exceeding value, and as century has followed century very great authority has been ascribed to them.