But there are two qualities which happen to be equal.

这里有两个质量恰好是相等的

耶鲁公开课 - 基础物理课程节选

There's no reason why these two attributes had to be proportional, but they are proportional and they are equal by choice of units and you can ask, "Is this just an accident or is it part of a big picture?"

我们不知道这两个质量为何成比例,但它们确实成比例,而且如果选择合适的单位还会相等,你会问,"这种情况纯属偶然,还是这只是物理学大体系的冰山一角"

耶鲁公开课 - 基础物理课程节选