Absolute co continuity of motion is not compre­hensible to the human mind. Laws of motion of any kind become comprehensible to man only when he examines arbitrarily selected elements of that moti­on; but at the same time, a large proportion of hu­man error comes from the arbitrary division of con­tinuous motion into discontinuous elements. There is a well-known, so-called sophism of the ancients consisting in this, that Achilles could never catch up with a tortoise he was following, in spite of the fact that he traveled ten times as fast as the tortoise.