Below is the first passage of the classic of entering medicine, originally by Cui Xifan, commented on here by Hun Ranzi, a Daoist master of the Yuan Dynasty period.
先天炁，後天炁。得之者，常似醉。
Pre heaven qi, post heaven qi, who grasps this will long seem drunk.
先天炁者，乃元始祖炁也。此祖炁在人身天地之正中，生門密戶懸中高處，天心是也。神仙修鍊，止是採取先天一炁，以為丹母。
Pre heaven qi is the energy of the original beginning. This original qi is in the exact centre of the heaven and earth inside of people’s bodies. Hanging from the high point of the life door, secet window. It is also the heart of the heavens.
Spiritual immortals who cultivate this should stop and pluck up the pre heaven one energy, which is also known as the mother of elixir.
後天炁者，乃一呼一吸，一往 一來，內運之炁也。呼則接天根，吸則接地根；呼則龍吟而雲起，吸則虎嘯而風生。綿綿若存，歸于祖炁內外混合，結成還丹。
Post heaven Qi is one out and one in breath, one going and one coming, the energy moving within.
Out meets with the root of the heavens. In meets with the root of the earth. Out is the dragon singing and riding on the clouds, in is the tiger growling on the gusting wind.
Gradually and without pause it is thus contained, returning to the original energy with inner and outer blended together. Thus it collects and changes to elixir.
自覺丹田火熾，暢於四肢。如癡如 醉，美在其中。此所以得之者常似醉也。《道德經》云：谷神不死，是謂玄牝。玄牝之門，是為天地根。綿綿若存，用之不勤。《易·坤卦》云：黃中通理，正位居 體，美在其中，而暢於四肢，如斯之謂也。
You should feel that the Dan Tian is being heated by flame, and mixing with the four limbs. As though dull and drunk, the beauty is in its centre. This is thus why it seems that one who has grasped it is in a drunken state.
The Dao De Jing says: the valley spirit never dies because of the mystery goddess. The gate of the mystery goddess is the root of heaven and earth. Continuously unbroken is it contained, its use is never forced.
The yijing earth trigram says: yellow’s centre is connected in principle, its true placement rests in the body. Its beauty is in the centre, and mixes togethe with the four limbs. This is what its (the text) meaning is supposed to be.