The Taoist text The Secret of the Golden Flower contains the following passages related to the practice of alchemy:
The work on the circulation of the light depends entirely on the backward-flowing movement, so that the thoughts (the place of heavenly consciousness, the heavenly heart) are gathered together. The heavenly heart lies between sun and moon (the two eyes).
Therefore when the light circulates, the energies of the whole body appear before its throne, as, when a holy king has established the capital and has laid down the fundamental rules of order, all the states approach with tribute; or as, when the master is quiet and calm, men-servants and maids obey his orders of their own accord, and each does his work.
Therefore you have only to make the light circulate: that is the deepest and most wonderful secret. The light is easy to move, but difficult to fix. If it is made to circulate long enough, then it crystallizes itself; that is the natural spirit-body. This crystallized spirit is formed beyond the nine heavens. It is the condition of which it is said in the Book of the Seal of the Heart: “Silently thou fliest upward in the morning”.
In carrying out this fundamental principle you need to seek for no other methods, but must only concentrate your thoughts on it. The book Leng Yen says: “By collecting the thoughts one can fly and will be born in heaven”. Heaven is not the wide blue sky but the place where corporeality is begotten in the house of the Creative. If one keeps this up for a long time there develops quite naturally, in addition to the body, yet another spirit-body.
The method used by the ancients for escaping from the world consisted in melting out completely the slag of darkness in order to return to the purely creative. This is nothing more than a reduction of the anima and a completion of the animus. And the circulation of the light is the magical means of reducing the dark, and gaining mastery over the anima.
If this method is followed,plenty of seed-water will be present of itself; the spirit fire will be ignited, and the thought-earth will solidify and crystallize. And thus the holy fruit matures. The scarabaeus rolls his ball and in the ball there develops life as the result of the undivided effort of his spiritual concentration. If now an embryo can grow in manure, and shed its shells, why then should not the dwelling place of our heavenly heart also be able to create a body if we concentrate the spirit upon it?
The radiation and dissipation of spiritual consciousness is chiefly brought about by this energy when it is directed outward (flows downward). Therefore the Way of the Golden flower depends wholly on the backward-flowing method.
In the book Leng Yen it is said: “By concentrating the thoughts, one can fly; by concentrating the desires, one falls”. When a pupil takes little care of his thoughts and much care of his desires, he gets into the path of submersion. Only through contemplation and quietness does true intuition arise: for that the backward-flowing method is necessary.
~ The Secret of the Golden Flower (source: http://www.lifeforcebooks.com/_Media/SecretGoldenFlower.pdf)