Avicenna defines elements as simple substances which provide the primary components of the human body. The four elements are earth, air, water and fire. These are symbolic expressions. Because all the elements recognized so far exceeding 100 should correspond to any of these four elements which virtually encompass all the known elements of today. These four elements have been assigned four temperamental qualities viz. hot, cold, moist and dry. With the existence of the matter in the universe, bodies also gain specific shape, and the quantum of matter a body contains is its mass. Since the body has to do some work, it should have energy to do so. Moist or dry states of matter shouId be charged with heat and cold in the form of energy. In theory mass and energy could be spelled out distinctly but in practice the two are inseparable. In view of the physical conditions as they are, we could conclude that heat is in fact the total kinetic energy possessed by the molecules of the body due to their rapid and random motion. Aristotle has rightly held the hot and cold as active, and dry and moist as passive qualities. This concept of element with heat and cold as two opposite types of energy and dryness and moisture as two opposite qualities of matter is the fundamental hypothesis which proceeds to explain the complicated phenomenon of human biology and psychology in physical terms.

    The four elements are earth and water (heavy), and fire and air (light). In terms of qualities, the heavy elements are strong, negative, passive, and female. The light elements are weak, positive, active, heavenly, and male.

    The chart below shows various correspondences for elements.

    Earth is an element usually situated at the center of our existence. In its nature it is at rest, and because of its inherent weight, all other elements gravitate toward it, however far away they may be. It is said to be cold and dry in nature, and it appears so to sight and touch, so long as it is not changed by any other elements. It is by means of the earth element that the parts of our bodies are fixed and held in place; thus the outward form of the body is due to the earth element.

    Water is a simple substance whose position in nature is exterior to the sphere of the earth and interior to that of air. Water is cold and moist in temperament, although only slightly so. Water is easily dispersed and assumes any shape without permanency. In the construction of "things," the addition of water allows the possibility of their being shaped and molded and spread out. Shapes can readily be made from it, and just as easily dispersed. Moisture dispels dryness, the latter being overruled by the former. Moisture protects dryness from crumbling (as moist earth, or mud), and likewise, dryness prevents moisture from dispersing. Thus the two elements of earth and water are interacting and interdependent. Water is, of course, absolutely essential to life.

    Air is positioned in nature above both water and earth, but beneath fire. The temperament of air is hot and moist, and its purpose in nature is to make things finer, lighter, and more delicate and thus more able to ascend into higher spheres. Air is also the agent by which breath moves in and out of the body and causes or makes possible the involuntary movements of the body.

    Fire is also a simple substance, situated higher than the other three elements. Fire is hot and dry in temperament, and its role in nature is to rarefy, refine, and intermingle things. Fire has the power to penetrate and can ride through the element of air. It has the capacity to overcome the coldness of the two cold elements, earth and water, and so creates and maintains harmony among the elements.

    Each of the elements has a corresponding humor or essence in the body: Blood humor relates to air. Phlegm humor relates to water. Yellow bile humor relates to fire. Black bile humor relates to earth.