 The final element is the role of correlations. To place phenomena in the same category is to situate them as having similar functions in analogous configurations. To be yin is to have a relationship to something that is yang, to be wood-like is to stand in certain relationships to fire, metal, earth, and air. This way of categorizing allows correlations across what would seem to be very different kinds of things. For example, an illness that is yang (expansive or overactive) can be treated with foods that are yin (receptive or calming). This kind of thinking is rooted in a concern for acting in harmony with natural patterns, going back to the concept of shi 時, which refers to the seasons, to the temporal configuration of a given moment, and to the ability to act according to the demands of that moment (Sellman 2002). These correlations were based on empirical observations, but as the categories were integrated and extended to all phenomena, the connections become less and less apparent, sometimes seeming forced and arbitrary. For example, for the first month that starts spring, the Lüshi chunqiu correlates certain days of the month, scaly animals, the musical note jue, sour tastes, and the number eight, but it also recommends surveying the land to set the boundaries of fields (so that conflicts are settled before planting begins), bans killing young animals and destroying eggs (so as to avoid shortages later), and forbids conscripting groups for war or major construction (so that they have time to plant the fields) (Chen Qiyou 1984, 1/1: 1–2). These are essentially rules for sustainability.