Chapter 9

Now since the left and the right sides are symmetrical, and these opposites are moved simultaneously¹, it cannot be that the left is moved by the right remaining stationary², nor vice³ 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 spinal⁴ 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 potency⁵ 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 spatial⁶ 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.

1 simultaneously   

adv.同时发生地，同时进行地

参考例句：

• The radar beam can track a number of targets almost simultaneously.雷达波几乎可以同时追着多个目标。
• The Windows allow a computer user to execute multiple programs simultaneously.Windows允许计算机用户同时运行多个程序。

2 stationary   

adj.固定的，静止不动的

参考例句：

• A stationary object is easy to be aimed at.一个静止不动的物体是容易瞄准的。
• Wait until the bus is stationary before you get off.你要等公共汽车停稳了再下车。

3 vice   

n.坏事；恶习；[pl.]台钳，老虎钳；adj.副的

参考例句：

• He guarded himself against vice.他避免染上坏习惯。
• They are sunk in the depth of vice.他们堕入了罪恶的深渊。

4 spinal   

adj.针的，尖刺的，尖刺状突起的；adj.脊骨的，脊髓的

参考例句：

• After three days in Japan,the spinal column becomes extraordinarily flexible.在日本三天，就已经使脊椎骨变得富有弹性了。
• Your spinal column is made up of 24 movable vertebrae.你的脊柱由24个活动的脊椎骨构成。

5 potency   

n. 效力，潜能

参考例句：

• Alcohol increases the drug's potency.酒精能增加这种毒品的效力。
• Sunscreen can lose its potency if left over winter in the bathroom cabinet.如果把防晒霜在盥洗室的壁橱里放一个冬天，就有可能失效。

6 spatial   

adj.空间的，占据空间的

参考例句：

• This part of brain judges the spatial relationship between objects.大脑的这部分判断物体间的空间关系。
• They said that time is the feeling of spatial displacement.他们说时间是空间位移的感觉。