When the middle-stage practice of "refining Qi into Spirit" yields results, one can further engage in the practice of "refining the Spirit and returning to Emptiness". "Refining the Spirit and returning to Emptiness" belongs to the "upper stage" of the "three-stage practice". Traditionally, "refining the Spirit and returning to Emptiness" takes nine years as the time marker for the completion of the alchemical process, so this level of practice is also called the "Nine-Year Barrier". In practice, "nine years" is not fixed but flexible, depending on various conditions such as the innate qualities, personal understanding, and external environment of the internal alchemy practitioner.

"Refining the Spirit and returning to Emptiness" is an advanced stage of internal alchemy cultivation and an ideal state of reaching the acme of perfection. If "refining Essence into Qi" is the practice of "gathering the three into two", and "refining Qi into Spirit" is the practice of "integrating the two into one", then "refining the Spirit and returning to Emptiness" is the process of bringing the Spirit to the extreme of emptiness.

Unlike the "active" operations of the initial stage and the "as if existing yet non-existing" operations of the middle stage, the cultivation of the upper stage is entirely a practice of "non-action". "Non-action" is the inherent nature of the Great Dao, which is inexpressible. Therefore, alchemists often use a circle "O" to represent it. This circle not only indicates that the mind has advanced to a state of emptiness but also symbolizes the all-encompassing and perfect nature of internal Qi.