看得见的球的运动轨迹的改变，就可能告诉我们：看不见的球是存在的。
The change in path of the visible ball would tell us about the presence of the invisible ball.
空气是看不见的，但有风时，我们能感觉到风的运动。
Air is invisible but we can feel its motion when there is a wind.
我们的生命是一种运动，一种趋势，是向一个看不见的目标无休止地奋进。
Our life is not a mere fact; it is a movement, a tendency, a steady, ceaseless progress towards an unseen goal.
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