 (1) Motions and actions may proceed as a result of volition and do so in a uniform and unvarying way, such as the motion of the heavenly bodies. The internal cause in these cases, Avicenna tells us, is a celestial  soul.[2]  (2) Other motions and actions may proceed as a result of volition but do so in a non-uniform and varying way, like the motions and actions of animals; the internal cause in these cases is an animal soul. (3) Again, other motions and actions may not proceed as a result of volition and are non-uniform and varying, like plant growth; the internal cause in these cases is a plant soul. Finally (4) some motions and actions may proceed without volition while doing so in a uniform and unvarying way, like the downward motion of a clod of earth and the heating of fire; the internal cause in these cases is a nature. 