This principle - that if two objects are in uniform motion relative to each other, it is impossible to decide which one is moving and which one is at rest - applies to all objects moving uniformly in a straight line through the universe.

　　如果两个物体在以相互间相对的同等的运动速度在运动，若想判定哪一个物体在动，哪一个物体原地不动，这是不可能的。这一原理适用于整个宇宙中凡是作直线等速运动的一切物体。

　　In relativityyou'll find that whenever you have a choice among things that are equally possible, you are dealing with relative things. For example, time, which is measured with clocks and watches, is relative because it can be shown that there is more than one system of time. All systems of time are equally possible and you can choose any system you with.

　　按着相对论的说法你就会发现，每当你在众多同样可能的事物中需要选定一种解释时，你就是在跟相对的事物打交道。例如，时间，虽然能用钟表加以测量，也是相对的，因为有不止一种时间体系可以来表示时间。所有的时间体系都同样有可能表示时间，所以你可以选用你所喜欢的时间体系来表示时间。