On The Motion Of Animals   

And it is plain that the power from which this force would originate

will not be infinite, for the earth is not infinite and therefore

its weight is not. Now there are more senses than one of the word

'impossible'. When we say it is impossible to see a sound, and when we

say it is impossible to see the men in the moon, we use two senses

of the word; the former is of necessity, the latter, though their

nature is to be seen, cannot as a fact be seen by us. Now we suppose

that the heavens are of necessity impossible to destroy and to

dissolve, whereas the result of the present argument would be to do

away with this necessity. For it is natural and possible for a

motion to exist greater than the force by dint of which the earth is

at rest, or than that by dint of which Fire and Aether are moved. If

then there are superior motions, these will be dissolved in succession

by one another: and if there actually are not, but might possibly be

(for the earth cannot be infinite because no body can possibly be

infinite), there is a possibility of the heavens being dissolved.

For what is to prevent this coming to pass, unless it be impossible?

And it is not impossible unless the opposite is necessary. This

difficulty, however, we will discuss elsewhere.

To resume, must there be something immovable and at rest outside

of what is moved, and no part of it, or not? And must this necessarily

be so also in the case of the universe? Perhaps it would be thought

strange were the origin of movement inside. And to those who so

conceive it the word of Homer would appear to have been well spoken:

'Nay, ye would not pull Zeus, highest of all from heaven to the

plain, no not even if ye toiled right hard; come, all ye gods and

goddesses! Set hands to the chain'; for that which is entirely

immovable cannot possibly be moved by anything. And herein lies the

solution of the difficulty stated some time back, the possibility or

impossibility of dissolving the system of the heavens, in that it

depends from an original which is immovable.

Now in the animal world there must be not only an immovable without,