Kundalini: Realise the soul not as matter, but as it is. We are thinking of the soul as body, but we must separate it from sense and thought. Then alone can we know we are immortal. Change implies the duality of cause and effect, and all that changes must be mortal. This proves that the body cannot be immortal, nor can the mind, because both are constantly changing. Only the unchangeable can be immortal, because there is nothing to act upon it.
We do not become it, we are it; but we have to clear away the veil of ignorance that hides the truth from us. The body is objectified thought. The "sun" and "moon" currents bring energy to all parts of the body. The surplus energy is stored at certain points (plexuses) along the spinal column commonly known as nerve centres.
These currents are not to be found in dead bodies and can only be traced in a healthy organism.
The Yogi has an advantage; for he is able not only to feel them, but actually to see them. They are luminous in his life, and so are the great nerve centres.
There is conscious as well as unconscious action. The Yogis possess a third kind, the superconscious, which in all countries and in all ages has been the source of all religious knowledge. The superconscious state makes no mistakes, but whereas the action of the instinct would be purely mechanical, the former is beyond consciousness.
It has been called inspiration, but the Yogi says, "This faculty is in every human being, and eventually all will enjoy it."
We must give a new direction to the "sun" and "moon" currents and open for them a new passage through the centre of the spinal cord. When we succeed in bringing the currents through this passage called "Sushumnâ", up to the brain, we are for the time being separated entirely from the body.
The nerve centre at the base of the spine near the sacrum is most important. It is the seat of the generative substance of the sexual energy and is symbolised by the Yogi as a triangle containing a tiny serpent coiled up in it. This sleeping serpent is called Kundalini, and to raise this Kundalini is the whole object of Raja-Yoga [Royal Science of Union].
The great sexual force, raised from animal action and sent upward to the great dynamo of the human system, the brain, and there stored up, becomes Ojas or spiritual force. All good thought, all prayer, resolves a part of that animal energy into Ojas and helps to give us spiritual power. This Ojas is the real man and in human beings alone is it possible for this storage of Ojas to be accomplished. One in whom the whole animal sex force has been transformed into Ojas is a god. He speaks with power, and his words regenerate the world.
The Yogi pictures this serpent as being slowly lifted from stage to stage until the highest, the pineal gland, is reached. No man or woman can be really spiritual until the sexual energy, the highest power possessed by man, has been converted into Ojas.
No force can be created; it can only be directed. Therefore we must learn to control the grand powers that are already in our hands and by will power make them spiritual instead of merely animal. Thus it is clearly seen that chastity is the corner-stone of all morality and of all religion. In Raja-Yoga especially, absolute chastity in thought, word, and deed is a sine qua non. The same laws apply to the married and the single. If one wastes the most potent forces of one's being, one cannot become spiritual.
Quoted from Swami Vivekananda, Lessons on Raja Yoga (1913)