On The Motion Of Animals   

original seat of the moving soul must be in that which lies in the

middle, for of both extremes the middle is the limiting point; and

this is similarly related to the movements from above [and below,]

those that is from the head, and to the bones which spring from the

spinal column, in creatures that have a spinal column.

And this is a reasonable arrangement. For the sensorium is in our

opinion in the centre too; and so, if the region of the original of

movement is altered in structure through sense-perception and thus

changes, it carries with it the parts that depend upon it and they too

are extended or contracted, and in this way the movement of the

creature necessarily follows. And the middle of the body must needs be

in potency one but in action more than one; for the limbs are moved

simultaneously from the original seat of movement, and when one is

at rest the other is moved. For example, in the line BAC, B is

moved, and A is the mover. There must, however, be a point at rest

if one is to move, the other to be moved. A (AE) then being one in

potency must be two in action, and so be a definite spatial

magnitude not a mathematical point. Again, C may be moved

simultaneously with B. Both the originals then in A must move and

be, and so there must be something other than them which moves but

is not moved. For otherwise, when the movement begins, the extremes,

i.e. the originals, in A would rest upon one another, like two men

putting themselves back to back and so moving their legs. There must

then be some one thing which moves both. This something is the soul,

distinct from the spatial magnitude just described and yet located

therein.

10



Although from the point of view of the definition of movement- a

definition which gives the cause- desire is the middle term or

cause, and desire moves being moved, still in the material animated

body there must be some material which itself moves being moved. Now