Between physics and metaphysics: Mulla Sadra on nature and motion

Islam & Science, June, 2003 by Ibrahim Kalin

The Mover and the Moving Body

We may remember that Aristotle had proposed the concept of the Prime Mover to terminate the infinite regression of causal chain. Put simply, if everything is moved by something else, this must end in an agent that itself does not move. An important consequence of this idea is the stark distinction between the mover and the moving body--a complementary duality that was extended in posterity to positional motion. Considered from the perspective of vertical causality, every moving body needs a mover, and Sadra, following the Peripetatics, reformulates this relationship in terms of actuality and potentiality. Since the process of motion requires the two poles of actuality and potentiality, actuality refers to the mover (al-muharrik), and potentiality to the moving body (al-mutaharrik). In other words, the mover as the actual being provides the cause of motion, and the moving body as the potential being stands at the receiving end of the process of motion.

This polarity shows that a single body cannot be both the active and passive agent of motion. In other words, we have to assume the existence of a prime mover to which all motion can ultimately be traced back. Sadra's argument runs as follows: The moving body, in so far as it is a potential being, has to be a passive agent, i.e., the receiver of the act of motion whereas the mover has to be an active agent, in so far as it is an actual being. These two qualities or 'aspects' (jihat) cannot be found in the same thing simultaneously due to their exclusive nature. In other words, a physical entity cannot be both the source and locus of motion at the same time. At this point, all motion should go back to an active agent which is

   different from motion as well as from the locus of
   motion, moving by itself, renewing itself by itself, and
   necessarily the source of all motion. And this (agent)
   has its own agent (i.e., principle) of motion in the sense
   of being the source of its own continuous renewal. By
   this, I do not mean the 'instaurer' (ja'il) of its motion
   because instauration cannot exist between a thing and
   itself. This is so because the direct agent of motion has
   to be something in motion. Otherwise this would
   necessitate the difference of the cause (al-'illah) from its
   effect (ma'luliha). Thus, if this (chain of causation) does
   not end in an ontological agent (amr wujudi) that
   renews itself by itself, this would lead to regression or
   circularity.' (21)

Sadra goes on to adduce proofs for the necessity of a prime mover as an external agent to set things in motion. He rejects and responds to some objections as follows. 1) If a thing were to move by itself, it would never reach rest because whatever endures by itself does so by its intrinsic qualities. Once these qualities or properties are disjoined from a thing, it no longer exists. 2) If a thing were to move by itself, parts of motion i.e. the subject of motion as a whole would be in rest, which means that the thing would not move. 3) If the principle of motion were to be in the moving body itself, it would have no 'fitting' or natural place to which it could incline. According to the conventional definition of motion, however, if there were to be no natural place for a thing to which it could incline, it could not move. 4) If self-motion were to be a real property of a moving body, it would be a universal quality of 'thing-ness' (shaybiyyah) shared by all corporeal things. But this is not the case in natural bodies. In reality, says Sadra, motion is a particular quality provided by an outside mover. 5) Another proof for the fact that a physical body cannot have the principle of motion in itself is that this would mean that both potentiality and actuality can be found at the same locus simultaneously. If this were the case, actuality would not be succeeded by potentiality. Because according to the definition given above, motion is the first perfection for what is potential. If a thing were able to move by itself, it would be actual in all respects without leaving any room for potentiality, which is obviously inconceivable for contingent beings. 6) The relation of the moving body to motion is established through contingency (bi'l-imkan), and its relation to motion as an active agent is necessary (bi'l-wujub). If the moving body itself were to be the producer of motion, this relation would be necessary. But since contingency and necessity cannot coincide, the moving body has to be different from the principle or source of motion. (22)