Non-intentional motion...

Non-intentional motion, in turn, is divided into natural and unnatural motion, for it is either required by the nature of the thing, or it is under the influence of a constraining force. Intentional motion, which is based on the soul of the willing agent, is really a subordinative ( taskhīrī ) action which would not occur without the intermediary of the soul.

The souls of animals and men use a natural agent to move their bodies or other objects, so the direct and proximate agent of intentional motion is nature. On the other hand, constrained motion, whether it derives from that which constrains ( qāsir ), as asserted by us, or from that which is constrained ( maqsūr ), as most philosophers have held, is ultimately produced by the nature of the body.

Hence, every motion derives from nature, and for this reason, nature is introduced as the agent source ( mabda’ fā’ilī ) of the motion of bodies. In other words, every motion has a source of its tendency ( mabda’ maylī ) which is either a property of the body’s nature or appears by means of the influence of the nature of another thing. The ancient philosophers presented views about the source of motion in moving bodies, some of which were discussed in Lesson Thirty-Eight.

However, their explanations were based on the assumptions of the then current natural sciences and do not correspond to contemporary scientific theories. But, in general, it can be said that corporeal motion does not lie beyond these two alternatives: either it is required by the nature of the moving existent—and in this case the motion continues until it confronts an obstacle—or the essence of the moving existent does not require motion, but it occurs under the influence of a foreign factor.

If this foreign factor itself does not require motion essentially, another factor will have to exist, until it culminates in a material factor that essentially requires motion. This factor corresponds to the thing which in modern physics is called ‘energy.’ It is the transference of energy to bodies which causes their motion. But it must be noted that the validity of this correspondence depends on the validity of the related scientific theory.

However, the existence of a natural factor that essentially requires motion is a philosophical theory to which the correctness or incorrectness of scientific theories makes no difference. Motion in Position Just about everything that has been said about spatial motion applies to motion in position as well.