415 it cannot be removed except by force: but it does not set out towards the center except by going down. Furthermore: what is moved naturally must be moved towards something determinate: but in what is upward, there is nothing of which we could say, This is the upward limit; on the contrary there can be infinite limits upward, and upward is extended without end: hence nothing can be moved naturally without end, towards the indeterminate -- hence, upward. Now concerning downward the same cannot be said: for there is a certain limit of the downward, one, finite, and even indivisible, from which a thing cannot be removed even by the width of a fingernail by seeking the downward; and such is the center. And do not tell me that there exists an upward limit, namely the concave sphere of the Moon {1}: for this is false. For the limit of a certain motion must be such that what recedes from it is no longer moved by the motion of which it was the limit: but the concave sphere of the Moon is not such; for the limit of upward motion is not such that what recedes from there cannot be moved further upward. But the center is so much the limit of downward motion that nothing, receding from there, can be moved downward more.
Remoteness from the center is infinite; but proximity is finite and determined by the center itself: hence if there is something endowed with the ability of fleeing from the center, surely this thing will be suited to be moved without end. What could be more absurd than this?
A motion to which a limit cannot be assigned cannot be natural: but to upward motion a limit cannot be assigned: hence upward motion is not natural. The minor is proved: a motion is delimited at a place from which, by the same motion, it is not possible to recede (for if by the same motion one could progress farther, the limit would not be there): but motion upward is nowhere delimited in such a way that from that place by the same motion, namely upward, it would be impossible to be brought farther: hence upward motion is nowhere delimited; hence it is deprived of a limit; hence it cannot be natural. Now it is evident that there is nowhere that a limit of upward motion can be placed, from which by the same motion it is impossible to be brought farther: for if any place is assigned [as a limit], it is possible to recede from it by going up, and another can be assigned which is more distant from the center than it. {1}