ABSTRACT
This chapter expresses Aristotle's view on the movement of animals, where he says that in animals if any part is moved, another must be at rest; this is why they have joints; animals use joints like a centre, and the complete part in which the joint is becomes both one and two, both straight and bent, changing potentially and actually by reason of the joint. He compares these movements with that of the mathematical concepts as when the joint is being bent and moved, one of the points in the joint is moved and one is at rest, just as if the points A and D on the diameter of a circle were at rest and B were moved so as to give the radius AC. However, in the latter case the centre is thought to be every way indivisible. This is referred to the Newton's Third Law of Motion, namely that action and reaction are equal and opposite.
