Aristotle's Account of the Joint

Aristotle believed that he had established that the “origin of other motions is that which moves itself, and that the origin of this is immovable, and that the prime mover must of necessity be immovable.” (Physics 698a 10) This proof was theoretical, and in the “Movement of the Animals” he wished to establish this concretely in the world of the senses, particularly in animal life.

Animals are unique in that they move themselves. Because animals initiate their own motion, they are both the one that causes movement and the one that moves. This distinction is important, for motion requires a division. What causes the motion and what suffers the motion must be distinct;. when an animal acts on itself then, there must be an active part which causes the motion, and a passive part which is moved. However the way in which the animal is divided in moving does not negate the way in which it is one, for when we say “the animal moves” we mean a singular thing by “animal.” The unity of an animal is more obvious than its distinct active and passive parts. When one sees a dog move, the evident thing is “the dog moved itself,” and it is only upon reflection that the active/passive distinction is made.

Why is the distinction between the active part of an animal and the passive part necessary? The source of the motion must itself be unmoved. When one considers the origin of a particular motion, this is origin is necessarily a beginning. The motion was not there, and then it was. The source of the motion could not have been in motion (participating in the same motion), for it must precede the motion. Therefore the origin of motion must necessarily be unmoved with respect to that motion. The active part of an animal is not in motion. Still it is the cause of the motion of something else, and so it “moves.” Is it possible to consider the active part and the passive part as the same, differentiated only temporally, in other words, one and the same part moved itself, and it is considered active before it moved itself and passive afterwards?

Aristotle provides an example to demonstrate the impossibility of such a proposition. If a man is trying to move a boat, this is done by standing outside the boat and pushing it while ones feet are on shore, or, if he remains in the boat, he applies force to the earth outside the boat in order to move the boat. He requires the stability of what is – in relation to the boat – immovable. Likewise, in animal motion, the part which moves requires a part at rest, against which it may be supported.

In the body, the juncture at which the moved meets the unmoved, which both divides a thing and unites it, is the joint. The joint contains a point from both the moved and unmoved, and provides the place in which the point of the moved moves, and the point of the immovable remains unmoved. It is the meeting of the two which participates in motion and yet abstains from it. Without the joint there is both no way in which the parts are divided so that one may be moved and the other unmoved, and there is no way in which the parts come together and, maintaining their distinctness, produce motion. Thus the forearm and the upper arm meet at the elbow, and it is against the resting upper arm that the forearm is able to move.

One initial objection to this account is inevitable. What about the obvious case where, for example, both the forearm and the upper arm moves when a boxer throws a punch? Obviously one part is able to derive its motion from another part, itself in motion. Here one must be careful of what is meant by motion. Motion is always motion of something. When considering the motion of, say, the forearm, the “rest” of the upper arm is considered in relation to the particular motion of the forearm, not the motion of the whole body. With respect to the forearm, the upper arm does not move, but with respect to the motion of a boxer's punch, the whole body moves, while the mat provides the support.

In the body then, the motion of a particular part is supported by an unmoving part, and the place in which the moving and unmoving meets is the joint. The motion has this structure: ABC, such that A is the moving part, B is the joint, and C is the unmoving part. Motions are considered on different levels, A may be the forearm in one motion, B the elbow, and C the upper arm in one motion; while in another A may be the arm, B the shoulder, and C the body. By keeping in mind that motion is always motion of a distinct part, the structure of bodily motion as consisting of the moved and unmoved meeting in the joint is easily discernible.