Taoism regards the cultivation of essence, qi (vital energy), and spirit as the fundamental approach. From the perspective of Taoists, in the primordial state of human life, essence, qi, and spirit exist as an integrated whole, which is why the infant stage is considered an ideal state. However, as time passes, humans are subjected to various disturbances, causing essence, qi, and spirit to separate into three. If this process continues, it will ultimately lead to death.

To avoid such an outcome, Taoists envision a reversal of time. In terms of human cultivation, this means "returning" to the state of an infant. The method of "returning" involves reuniting the separated essence, qi, and spirit.

The first step in this reunification is "refining essence into qi," also known as "gathering the three into two." Here, "three" refers to "essence, qi, and spirit," while "two" refers to "qi and spirit." Since "essence" is refined into qi, "essence" becomes formless, leaving only "qi and spirit." Generally, refining essence into qi requires one hundred days, hence it is also called the "Hundred-Day Barrier." As this step is the initial stage among the three major stages of cultivating essence, qi, and spirit, it is also known as the "Primary Barrier."

According to the cultivation process, refining essence into qi belongs to the category of "active" practices. The so-called "active" means it requires a series of steps to achieve the intended goal. These steps are: collecting the medicine, sealing and securing, simmering and refining, and stopping the fire. These four major steps, metaphorically derived from the smelting process of external alchemy, generally involve initiating the innate primordial qi through meditation, sending it to the lower dantian for storage, then using one's own intention to observe and illuminate, prompting the essence to transform into qi, which circulates along the Ren and Du meridians. When qi and spirit 凝聚 (coalesce), it is time to stop the fire, thus achieving the effect of refining essence into qi.