Swami Vivekananda translated into English the aphorisms of Patanjali which he called Raja Yoga or the science of concentration. In his translation he followed the orthodox commentaries, but in many places the ideas were his own. This abstruse subject he made easy to understand and vital withal. Of all his Yoga writings, his book , on Raja Yoga is most popular with many, at least in the West.
He brings out the subject of Raja Yoga from mystical darkness to the bright light of day and deplores how this grand, rational and practical science has been made into an art of magic by mystery-mongers. His explanations of Prajna, Akasha, Kundalini, Shat-chakra, the control of Psychic Prajna are original and marvellous. Here he spoke, one can easily see, from his direct realizations and not from intellect. It is a book of guidance and directions and not of explanation or psychology. And his side remarks are now and then highly inspiring and immensely helpful to one's daily life. For instance, why and how does concentration of the mind lead to the realization of the Self? Why is love all-powerful? How does hatred cause more harm to the person from whom it emanates than to whom it is done?
To quote his own words: '... just as in the case of electricity, the modern theory is that the power leaves the dynamo and completes the circle back to the dynamo, so with hate and love; they must come back to the source. Therefore do not hate anybody, because that hatred which comes out from you, must, in the long run, come back to you.
If you have love, that love will come back to you, completing the circuit. It is as certain as can be, that every bit of hatred that goes out of the heart of a man comes back to him in full force, nothing can stop it; similarly every impulse of love comes back to him.' Many such practical directions can be found. It is said that a place of pilgrimage reawakens when a man of God visits the place. This is how the sacred places remain the source of spiritual inspiration for ages. The scriptures also become living and revitalized when a man of God throws the light of his experiences on them. Swami Vivekananda brought new life to our old and ancient scriptures.