The Motion in a Right Line Must Stop

It is impossible for the motion in a right line to be continued because that which returns must necessarily stop if it is moved upon a circle. Since there are three things the beginning middle and end the middle compared with each of these is both and in number indeed it is one but also an energy. Any point whatever of a right line which is between the extremes is in capacity indeed the middle but not in energy unless it divides the line and having stood still again begins to be moved. That which was the middle becomes the beginning and end the beginning indeed of the posterior but to the end of the prior motion I say.