"The mind is an instrument, as it were, in the hands of the soul, through which the soul catches external objects. The mind is constantly changing and vacillating, and can, when perfected, either attach itself to several organs, to one, or to none. For instance, if I hear the clock with great attention, I will not, perhaps, see anything although my eyes may be open, showing that the mind was not attached to the seeing organ, while it was to the hearing organ. But the perfected mind can be attached to all the organs simultaneously. It has the reflexive power of looking back into its own depths. This reflexive power is what the Yogi wants to attain; by concentrating the powers of the mind, and turning them inward, he seeks to know what is happening inside."

- Swami Vivekananda, "Introductory" (Dec 14, 1895, New York Vedanta Society), Raja-Yoga, Complete Works: Vol 1