QUOTE : However, the four-elements concept is a different system of classification of matter than is our periodic table of elements; it is based on the physical state (solid, liquid, gas, energy), acceptance or rejection of moisture (wet, dry), acceptance or rejection of heat (hot, cold), and relationship to other elements (inner, middle, outer, mixed). Why is such a classification needed? The answer is simple: because it is compatible with the biological nature of living organisms. The physical state, heat, and water are three criteria that can describe the conditions of a biological entity—organs, structures, biochemical compounds, liquids, and such.