 Additionally, Avicenna specifies different kinds of elements based upon three kinds of natural motions. The three kinds of natural motion are (1) motion around the center, (2) motion toward the center and (3) motion away from the center (Avicenna, [DC], 2). Celestial bodies are in turn identified with those bodies whose natural motion is around the center. Terrestrial bodies are identified with those undergoing rectilinear motion either toward or away from the center. By convention, what moves away from the center moves upward, while what moves toward the center moves downward. Avicenna believes that moving around the center, circular motion, is generically simple inasmuch as he thinks that there is no contrary to circular motion (Avicenna, [Ph], 4.6 [7–11] &amp; [DC],  2).[8]  Consequently, the very nature of a celestial body is to move circularly (Avicenna, [PR], namaṭ 2, ch. 12). As for the terrestrial bodies, moving upward and downward are also generic descriptions; however, Avicenna continues, each of these genera also has two species, corresponding with the different termini of the motion. Thus, for example, those bodies that move downward are generically alike in that they move away from the surrounding celestial sphere; however, they can differ in that some have the center as their proper place or terminus for their motion, whereas the terminus of other downward-moving bodies falls short of the center. Any body that has the center as its proper place has absolute downward motion, while a downward-moving body whose terminus falls short of the center has relative downward motion. The same holds mutatis mutandis for bodies that move upward. Those that have the limit of the surrounding celestial spheres as their terminus have absolute upward motion, while those whose terminus falls short of that limit have relative upward motion. 