It is on this principle that Mantra Yoga was founded. Words which sprang from the intuition of the Yogis and thinkers, words which conveyed the meaning in a most profound manner, such words were collected for the use of the adepts, who repeated them and who profited by repeating them. Mantra Yoga means a science of words, words which were sacred and helpful in one's spiritual evolution. The Yogis have worked on this principle for many thousands of years, and have discovered a great mystery in the power of words. Sufis of all ages have followed this principle of making use of words which are suggestive of a certain sense, a sense which one wishes to bring out and make a reality in one's life. No doubt it is necessary to know the meaning of the sacred words one repeats; this gives a thousandfold greater effect. And the spoken word has a greater power than silent concentration, provided there is power of concentration and sincere feeling at the back of that word.