Aristotle's treatise On the Movement of Animals argues that the movement of animals – as every other type of locomotion – exemplifies the general kinetic scheme according to which all movement requires (i) an unmoved element, (ii) something that imparts movement while being moved and (iii) a moved body. Aristotle is eager to show that for the initiation of the movement of an animal the unmoved element comes in two versions: (a) as an external resting point against which the animals must support itself and (b) as an inner resting point within the joints of an animal. The mechanism of the joint is of particular interest for Aristotle, because it always has to include both a moved and an unmoved part. This is the model he invokes in the course of his answer to the question of how the soul moves the body.
The talk tries to make sense of this model of the joint within Aristotle's psychophysical account of animal movement.