Yoga works on the level of one's body, mind, emotion and energy. According to this, four broad categories of Yoga exist: Karma Yoga impacts body; Jnana Yoga impacts mind; Bhakti Yoga impacts emotion and Kriya Yoga impacts energy. Each Yoga system used in practice embraces one or more of these categories. Every individual is a unique combination of these four factors. Only a guru (teacher) can advocate the appropriate combination of the four fundamental paths as is necessary for each seeker. All ancient scripts on Yoga point out that it is essential to practice under the direction of a guru.