On the Movement of Animals, Part Two

Chap. 1. Concerning the modes and mechanical operations by which the contraction of muscles can occur.

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that is, from nothing, the contractive force of the muscle cannot be destroyed. Therefore, upon the deficiency of that appetite, there would be present an impeding cause which would act by destroying the contractive force, and afterwards, upon the arrival of that same desire, it would destroy that impediment. But this is to multiply entities in vain and without necessity. Therefore, without any impeding cause, the positing of a single cause, which can contract the muscles at the command of the appetitive faculty, is sufficient.

PROPOS. VII.

I know that some believe that water consists of certain little rods which, while they run about in a winding manner like eels among tangled masses composed of the same fibrils, render the water fluid; but when they stiffen from cold and remain entirely immobile, they then acquire the hardness of ice. In the same way, they think that the mere rest of the component parts is the most sufficient cause for the consistency and firmness of all hard bodies. But because muscles, when they act, are tensed and hardened, they do not fear to affirm that the faculty of the soul, in commanding rest within the muscles, creates tension and rigidity in them.

a Prop. 143

However, such an imagination appears to be false, as we showed in the book concerning natural motions dependent upon gravity . Furthermore, in our case, granting that muscular fibers might acquire tension and hardness from the rest of their parts,