"Fire Phases"( 火候 Huohou ) is a term of the science of the Golden Elixir ( 金丹 Jindan ). The theory of "Fire Phases" develops from the refining practice of the External Alchemy Skill of Refining the Yellow and the White ( 金丹黃白術 Jindan Huangbaishu ) and refers to the skill of controlling fire, procedure, temperature changes and the like in the refining process. In accordance with the principle of the Oneness of Heaven and Man ( 天人合一 Tianren Heyi ), through their practice and experience, Daoists created the procedures of manipulating fire phases, the general guideline of which is to control the increasing and decreasing of fire phases in imitation of the alternation of the sun and the moon and that of Yin and Yang. This theory was also introduced into the practice of Inner Alchemy ( 內丹 Neidan ). From the point of view of Inner Alchemy, "Fire Phases" are in fact a metaphor, which signify the degrees of exercising the mind and breathing. In alchemists’ eyes, the mastery of Fire Phases is so important that it can be even regarded as the decisive factor in the refinement of the Golden Elixir. There has always been a popular statement in Daoism that "it is easy to get familiar with elixir drugs, but it is difficult to get the accurate mastery of fire phases", so we must act cautiously. Inner Alchemist Liu Yiming points out in Direct Pointers on Realizing Perfection ( 悟真直指 Wuzhen Zhizhi ) that "fire" refers to the efficacy of Religious Practice ( 修持 Xiuchi ), while "phases" refer to its sequence. There are various fire phases, such as that of gentle refinement, that of strong refinement, that of initiation, that of termination, that of Reversion Elixir ( 還丹 Huandan ), that of the great elixir, that of increase and decrease, that of Nourishment in Warmth ( 溫養 Wenyang ), etc. The key to Fire Phases is mind and thoughts. "Thoughts" refer to the miscellaneous distracting ideas, while "mind" refers to one’s own "spirit" that controls breathing. Improper ideas shouldn’t come forth; if so, they will cause hot fire. One’s spirit shouldn’t be distracted; if so, they will cause cold fire. Therefore, one should find out the state of his mind and investigates the heat of fire, or symbolically speaking, imitate changes in the nature. For example, a day goes through changes between Zi, Wu, You and Mao; a year goes through the alternation of Hai and Zi. Only with knowledge of the vicissitudes of Yin and Yang can one change with the situation and make great elixirs.