"In acquiring knowledge we make use of generalizations, and generalization is based upon observation. We first observe facts, then generalize, and then draw conclusions or principles. The knowledge of the mind, of the internal nature of man, of thought, can never be had until we have first the power of observing the facts that are going on within. It is comparatively easy to observe facts in the external world, for many instruments have been invented for the purpose, but in the internal world we have no instrument to help us. Yet we know we must observe in order to have a real science. Without a proper analysis, any science will be hopeless - mere theorising... The science of Raja-Yoga, in the first place, proposes to give us such a means of observing the internal states. The instrument is the mind itself. The power of attention, when properly guided, and directed towards the internal world, will analyse the mind, and illumine facts for us."

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