Fire, huo, is one of the five phases of the Wu Xing, and it should not be simply equated with the fire of the Greek four-element scheme, since the five phases are agents in cyclic relation rather than material building-blocks. Fire is associated with summer, the south, the colour red, the planet Mars and the quality of maximum yang, heat, brightness, upward and outward movement at its peak. In the generating cycle fire is produced by wood and gives rise to earth (fire leaves ash, which becomes soil); in the controlling cycle fire is subdued by water and itself melts and controls metal.
In Chinese medicine fire governs the heart and small intestine, the emotion of joy, the tongue and the pulse, and it is linked to the spirit, shen, that resides in the heart. Fire's symbolism is of warmth, passion, illumination and the fullness of expression, the phase of ripeness and radiance; its virtue is propriety and its excess is agitation. It stands at the yang peak of the cycle, the moment of greatest activity before the turn toward earth and decline.
Sources Science and Civilisation in China, Volume 2: History of Scientific ThoughtJoseph Needham with Huangdi Neijing (The Yellow Emperor's Inner Classic)Traditionally attributed to Huangdi; actual authorship unknown