It is a fact that mysticism cannot be defined in words in the form of doctrines, theories, or philosophical statements, for mysticism is an inner experience. In order to know an inner experience one must arrive at that experience. If we say to a person who has never had a headache in his life, "I have a headache," he will never understand it; he will not know what it is. Therefore the word mysticism means nothing; it is through the inner experience that one realizes its meaning in all its fullness. Naturally, therefore, we can find many books on psychology or philosophy, but seldom anything on mysticism; and the few there are on mysticism are generally about something quite different from what mysticism really is. The reason is that it cannot be put into a book; it cannot be expressed in words. But the very reason why it is so vague is why it so valuable, for if there is any knowledge that is worth while, if there is any science which is precious, it is the knowledge and science which one can get out of one's mystical experiences.