The four elements of the sublunar world are, fire, air, water, earth. In their purity these elements have neither color nor taste, nor odor nor any other sensible property. For the elements are simple bodies, whereas the sensible qualities are the result of the composition of the elements. If air had color, we should see it as we see all colored things; and all other things would appear to us in the color of air, as is the case when we look through a colored glass. The same argument applies to water. The elements change into each other. We see water changing under the effect of heat into vapor, and the vapor condenses again under the influence of cold and changes back to water, namely, rain. Air changes into fire when flint strikes iron. Fire cannot exist here unless it has something to take hold of; otherwise it changes into air. Earth and water change into each other very slowly, because earth is hard to change. The basis of the four elements is a substance filling place as a result of its assuming the form of corporeality, _i. e._, extension in three directions. Filling place, it moves; moving, it becomes warm. When its motion is completed, it necessarily comes to rest and becomes cold. Heat and cold are the active powers, wet and dry are the passive qualities, wet being associated with heat, dry with cold. The mixture of these qualities with the corporeal basis results in the four elements. The three natures, mineral, plant, animal are composed of the four elements. When a seed is put in the ground it cannot grow without water, and sunshine and air. These form its food, and food is assimilated to the thing fed. Our bodies are composed of the four elements, because they are nourished by plants. The general process of the sublunar world is that of genesis and dissolution. The genesis of one thing is the dissolution of another. The dissolution of the egg is the genesis of the chicken; the dissolution of the chicken is the genesis of the four elements; for in the living being the elements are potential, and they become actual when the animal dies. This continuous process of genesis and dissolution proves that this world is not permanent, for the basis of its processes is change.[170]