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its elbow…joint would be situate the original seat of the moving soul。
Since； however； it is possible for a lifeless thing to be so related
to the hand as the forearm is to the upper （for example； when a man
moves a stick in his hand）； it is evident that the soul； the
original of movement； could not lie in either of the two extreme
points； neither； that is； in the last point of the stick which is
moved； nor in the original point which causes movement。 For the
stick too has an end point and an originative point by reference to
the hand。 Accordingly； this example shows that the moving original
which derives from the soul is not in the stick and if not； then not
in the hand； for a precisely similar relation obtains between the hand
and the wrist； as between the wrist and the elbow。 In this matter it
makes no difference whether the part is a continuous part of the
body or not； the stick may be looked at as a detached part of the
whole。 It follows then of necessity that the original cannot lie in
any individual origin which is the end of another member； even
though there may lie another part outside the one in question。 For
example； relatively to the end point of the stick the hand is the
original； but the original of the hand's movement is in the wrist。 And
so if the true original is not in the hand； be…there is still
something higher up； neither is the true original in the wrist； for
once more if the elbow is at rest the whole part below it can be moved
as a continuous whole。
9
Now since the left and the right sides are symmetrical； and these
opposites are moved simultaneously； it cannot be that the left is
moved by the right remaining stationary； nor vice versa； the
original must always be in what lies above both。 Therefore； the
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
that which is moved； but whose nature is not to initiate movement；
is capable of being passive to an external force； while that which
initiates movement must needs possess a kind of force and power。 Now
experience shows us that animals do both possess connatural spirit and
derive power from this。 （How this connatural spirit is maintained in
the body is explained in other passages of our works。） And this spirit
appears to stand to the soul…centre or original in a relation
analogous to that between the point in a joint which moves being moved
and the unmoved。 Now since this centre is for some animals in the
heart； in the rest in a part analogous with the heart； we further
see the reason for the connatural spirit being situate where it
actually is found。 The question whether the spirit remains always
the same or constantly changes and is renewed； like the cognate
question about the rest of the parts of the body； is better postponed。
At all events we see that it is well disposed to excite movement and
to exert power； and the functions of movement are thrusting and
pulling。 Accordingly； the organ of movement must be capable of
expanding and contracting； and this is precisely the characteristic of
spirit。 It contracts and expands naturally； and so is able to pull and
to thrust from one and the same cause； exhibiting gravity compared
with the fiery element； and levity by comparison with the opposites of
fire。 Now that which is to initiate movement without change of
structure must be of the kind described； for the elementary bodies
prevail over one another in a compound body by dint of
disproportion； the light is overcome and kept down by the heavier； and
the heavy kept up by the lighter。
We have now explained what the part is which is moved when the
soul originates movement in the body； and what is the reason for this。
And the animal organism must be conceived after the similitude of a
well…governed commonwealth。 When order is once established in it there
is no more need of a separate monarch to preside over each several
task。 The individuals each play their assigned part as it is
ordered； and one thing follows another in its accustomed order。 So
in animals there is the same orderliness… nature taking the place of
custom… and each part naturally doing his own work as nature has
composed them。 There is no need then of a soul in each part； but she
resides in a kind of central governing place of the body； and the
remaining parts live by continuity of natural structure； and play
the parts Nature would have them play。
11
So much then for the voluntary movements of animal bodies； and the
reasons for them。 These bodies； however； display in certain members
involuntary movements too； but most often non…voluntary movements。
By involuntary I mean motions of the heart and of the privy member；
for often upon an image arising and without express mandate of the
reason these parts are moved。 By non…voluntary I mean sleep and waking
and respiration； and other similar organic movements。 For neither
imagination nor desire is properly mistress of any of these； but since
the animal body must undergo natural changes of quality； and when
the parts are so altered some must increase and other decrease； the
body must straightway be moved and change with the changes that nature
makes dependent upon one another。 Now the causes of the movements
are natural changes of temperature； both those coming from outside the
body； and those taking place within it。 So the involuntary movements
which occur in spite of reason in the aforesaid parts occur when a
change of quality supervenes。 For conception and imagination； as we
said above； produce the conditions necessary to affections； since they
bring to bear the images or forms which tend to create these states。
And the two parts aforesaid display this motion more conspicuously
than the rest； because each is i