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Both of these wheel-shaped diagrams resemble in their structure the diagram of the microcosmic-macrocosmic harmony on the preceding page of this manuscript. The eight intersecting arcs of the top diagram show the relationships among the four seasons, the four qualities of the year, the four cardinal directions, and the dates of seasonal changes. Thus, this diagram illustrates the notion of the unity of time and space as expressed in the Spanish scholar Isidore of Seville's (d. 636 CE) scientific work, De natura rerum (On the nature of things, X). The bottom diagram illustrates the relationships among the four elements – Earth, Air, Fire, and Water – the four seasons, and the four bodily humors – phlegm, blood, yellow bile, and black bile -- as well as giving qualities associated with each. These relationships, first articulated by classical authorities and reprised in Isidore's De natura rerum, XI, iii, form the basis of medieval medicine.