From earliest antiquity, the Chinese viewed the cosmos holistically, with all of existence based upon one reality or "substance":  the primordial breath or qi.  

Qi rises and falls, changes and transforms in accordance with observable patterns and principles, which are themselves changing and evolving as the universe evolves.  Its "rising and falling" is dependent upon its own transformations and inner make-up (not directed or altered by an external creator).  The "light and airy" qi -- gaseous, Heaven-bound, spiritual -- rises up to form the heavens, gods and constellations.  The "turbid, heavy" qi -- solid, Earth-bound, material -- congeals downwards to form the earth, humans and "ten thousand things".  Thus, qi describes a single reality, a spiritual-material continuum (in contrast to the dualistic cosmology of the West), and is governed by a self-directed, cosmic "way of things" built into the very nature of existence itself.  

This cosmic way is the Dao, at once constant and yet ever changing, primordial and yet still coming into existence.  (Though later adopted by the Daoists to name their "pattern of thought", Dao is a pan-Chinese term, employed by Confucians, Daoists, and Buddhists alike.)  The etymology of the Chinese character Dao suggests a path made by running beasts -- that is, a path made naturally, formed by continual use.  Thus, the Dao is the natural path of all things, all manifestations of qi.  

Qi has two aspects, corresponding to the "inhalations" and "exhalations" of cosmic breath.  These are yin and yang, passive and active manifestations of qi, both essential to the health and harmony of existence.  Just as an organism cannot survive without both inward and outward breath, oscillating in intensity but always tending towards the other, the universe cannot survive without the balance and harmony of yin and yang.  Notice how the movement of yin and yang accounts for the existence of all potentialities and all things.