Unfortunately for non-Sanskrit-knowing people, the literature of Yoga is not largely available in English. The general teachings of Yoga are to be found in the Upanishads, and the Bhagavad-Gita; those, in many translations, are within your reach, but they are general, not special; they give you the main principles, but do not tell you about the methods in any detailed way. Even in the Bhagavad-Gita, while you are told to make sacrifices, to become indifferent, and so on, it is all of the nature of moral precept, absolutely necessary indeed, but still not telling you how to reach the conditions put before you. The special literature of Yoga is, first of all, many of the minor Upanishads, "the hundred-and-eight" as they are called. Then comes the enormous mass of literature called the Tantras. These books have an evil significance in the ordinary English ear, but not quite rightly. The Tantras are very useful books, very valuable and instructive; all occult science is to be found in them. But they are divisible into three classes: those that deal with white magic, those that deal with black magic, and those that deal with what we may call grey magic, a mixture of the two. Now magic is the word which covers the methods of deliberately bringing about super-normal physical states by the action of the will.
A high tension of the nerves, brought on by anxiety or disease, leads to ordinary hysteria, emotional and foolish. A similarly high tension, brought about by the will, renders a man sensitive to super-physical vibrations Going to sleep has no significance, but going into Samadhi is a priceless power. The process is largely the same, but one is due to ordinary conditions, the other to the action of the trained will. The Yogi is the man who has learned the power of the will, and knows how to use it to bring about foreseen and foredetermined results. This knowledge has ever been called magic; it is the name of the Great Science of the past, the one Science, to which only the word " great " was given in the past. The Tantras contain the whole of that; the occult side of man and nature, the means whereby discoveries may be made, the principles whereby the man may re-create himself, all these are in the Tantras. The difficulty is that without a teacher they are very dangerous, and again and again a man trying to practice the Tantric methods without a teacher makes himself very ill. So the Tantras have got a bad name both in the West and here in India. A good many of the American " occult " books now sold are scraps of the Tantras which have been translated. One difficulty is that these Tantric works often use the name of a bodily organ to represent an astral or mental centre. There is some reason in that because all the centres are connected with each other from body to body; but no reliable teacher would set his pupil to work on the bodily organs until he had some control over the higher centres, and had carefully purified the physical body. Knowing the one helps you to know the other, and the teacher who has been through it all can place his pupil on the right path; but it you take up these words, which are all physical, and do not know to what the physical word is applied, then you will only become very confused, and may injure yourself. For instance, in one of the Sutras it is said that if you meditate on a certain part of the tongue you will obtain astral sight. That means that if you meditate on the pituitary body, just over this part of the tongue, astral sight will be opened. The particular word used to refer to a centre has a correspondence in the physical body, and the word is often applied to the physical organs when the other is meant. This is what is called a " blind," and it is intended to keep the people away from dangerous practices in the books that are published; people may meditate on that part of their tongues all their lives without anything coming of it; but if they think upon the corresponding centre in the body, a good deal much harm may come of it. " Meditate on the navel," it is also said. This means the solar plexus, for there is a close connection between the two. But to meditate on that is to incur the danger of a serious nervous disorder, almost impossible to cure. All who know how many people in India suffer through these practices, ill-understood, recognize that it is not wise to plunge into them without some one to tell you what they mean, and what may be safely practiced and what not. The other part of the Yoga literature is a small book called the sutras of Patanjali. That is available, but I am afraid that few are able to make much of it by themselves. In the first place, to elucidate the Sutras, which are simply headings, there is a great deal of commentary in Sanskrit, only partially translated. And even the commentaries have this peculiarity, that all the most difficult words are merely repeated, not explained, so that the student is not much enlightened.