the earth appear to have based the fable upon intelligent grounds.

They make Atlas a kind of diameter twirling the heavens about the

poles. Now as the earth remains still this would be reasonable enough,

but their theory involves them in the position that the earth is no

part of the universe. And further the force of that which initiates

movement must be made equal to the force of that which remains at

rest. For there is a definite quantity of force or power by dint of

which that which remains at rest does so, just as there is of force by

dint of which that which initiates movement does so; and as there is a

necessary proportion between opposite motions, so there is between

absences of motion. Now equal forces are unaffected by one another,

but are overcome by a superiority of force. And so in their theory

Atlas, or whatever similar power initiates movement from within,

must exert no more force than will exactly balance the stability of

the earth- otherwise the earth will be moved out of her place in the

centre of things. For as the pusher pushes so is the pushed pushed,

and with equal force. But the prime mover moves that which is to begin

with at rest, so that the power it exerts is greater, rather than

equal and like to the power which produces absence of motion in that

which is moved. And similarly also the power of what is moved and so

moves must be greater than the power of that which is moved but does

not initiate movement. Therefore the force of the earth in its

immobility will have to be as great as the force of the whole heavens,

and of that which moves the heavens. But if that is impossible, it

follows that the heavens cannot possibly be moved by any force of this

kind inside them.

4



There is a further difficulty about the motions of the parts of

the heavens which, as akin to what has gone before, may be

considered next. For if one could overcome by force of motion the

immobility of the earth he would clearly move it away from the centre.