Some sages are also considered to have become one of the immortals (xian) through their mastery of the Dao. After shedding their mortal form, spiritual immortals may have many superhuman abilities like flight and are often said to live in heavenly realms. The sages as thus because they have attained the primary goal of Daoism: a union with the Dao and harmonization or alignment with its patterns and flows. This experience is one of being attuned to the Dao and to our own original nature, which already has a natural capacity for resonance (ganying) with Dao. This is the main goal that all Daoist practices are aiming towards and can be felt in various ways, such as a sense of psychosomatic vitality and aliveness as well as stillness and a "true joy" (zhenle) or "celestial joy" that remains unaffected by mundane concerns like gain and loss. Cosmology Daoist cosmology is cyclic—the universe is seen as being in constant change, with various forces and energies (qi) affecting each other in different complex patterns. Daoist cosmology shares similar views with the School of Naturalists. Daoist cosmology focuses on the impersonal transformations (zaohua) of the universe which are spontaneous and unguided. Livia Kohn explains the basic Daoist cosmological theory as follows: the root of creation Dao rested in deep chaos (ch. 42). Next, it evolved into the One, a concentrated state cosmic unity that is full of creative potential and often described in Yijing terms as the Great Ultimate (Taiji). The One then brought forth “the Two,” the two energies yin and yang, which in turn merged in harmony to create the next level of existence, “the Three” (yin-yang combined), from which the myriad beings came forth. From original oneness, the world thus continued to move into ever greater states of distinction and differentiation. The main distinction in Daoist cosmology in that between yin and yang, which applies to various sets of complementary ideas: bright - dark, light - heavy, soft - hard, strong - weak, above - below, ruler - minister, male - female, and so on. Cosmically, these two forces exist in mutual harmony and interdependence. Yin and yang are further divided into five phases (Wu Xing, or five materials): minor yang, major yang, yin/yang, minor yin, major yin. Each of these is correlated with a specific substance: wood, fire, earth, metal, water respectively. This schema is used in a variety