 In order to explain the specific differences of the terrestrial elements, Avicenna, following a long tradition, which can be traced back at least as far as Hippocrates (460–370 BCE), appeals to certain basic qualitative powers. These qualitative powers are hot/cold, which are seen as active powers following upon the form, and wet/dry, which are seen as passive powers, following upon the matter (Avicenna, [DC], 1). Additionally, Avicenna claims, the downward motion of those elements so naturally inclined, namely, earth and water, is the result of their being qualitatively cold and being directed to their natural places, that is, toward the center. Analogously, the upward motion of those elements so naturally inclined, namely, fire and air, is the result of their being qualitatively hot and being directed to their natural places, that is, away from center. The elements, earth and water’s natural inclination (mayl) downward is referred to as their heaviness, while fire and air’s natural inclination upward references their lightness (Avicenna, [DC], 2). As for the qualities, wet and dry, these passive powers refer to a body’s susceptibility or lack thereof to the impressions and actions of some form. For example, what is wet or fluid more easily receives the impression of a form (just think of wet clay). In contrast, what is dry, while more readily retaining any impressions made in it, does not easily yield to being impressed (now think of kiln-dried clay). Given these differences in motions and qualitative powers, Avicenna, again following in this tradition, identifies four terrestrial elements: (1) fire, which is hot and dry with an absolute upward motion, (2) air, which is hot and wet with a relative upward motion, (3) water, which is cool and wet with a relative downward motion and (4) earth, which is cool and dry with an absolute downward motion. Finally, and for completeness, (5) celestial bodies are neither hot nor cold (and so neither heavy nor light) nor wet nor dry, but instead are solely identified in term of their natural circular motion. 