It's so true it's like the wind. You cannot see it, but you can feel it."

VOA: standard.2010.04.05

Harmony is indeed invisible in sense Number 1, you cannot see it with your eyes, but for all that, it can be destroyed.

和声用无形的第一种解读方式理解，确实是无形的，你看不见它，但是它可以毁灭。

耶鲁公开课 - 死亡课程节选

And in terms of doing this for your homework, I actually want to mention that in the back page of your notes I attached a periodic table that does not have electron configurations on them. It's better to practice doing electron configurations when you cannot actually see the electron configurations.

关于你们的作业，是在你们讲义的后面的页,我附上了一张,没有给出电子构型的周期表,最好联系一下做出电子构型,当你们实际上看不到,电子构型的时候。

麻省理工公开课 - 化学原理课程节选

You cannot say to me, "Take a force, due to a spring, and see what force it applies and divide by the acceleration and get the mass, " because we haven't defined force, either.

你不能说,"给弹簧施加一个力,看看它能提供多大的力,然后除以加速度,得到质量",因为我们还没给力下一个定义

耶鲁公开课 - 基础物理课程节选